4-Hydroxynonenal signalling to apoptosis in isolated rat hepatocytes: the role of PKC-delta.

4-Hydroxynonenal, a significant aldehyde end product of membrane lipid peroxidation with numerous biochemical activities, has consistently been detected in various human diseases. Concentrations actually detectable in vivo (0.1-5 microM) have been shown to up-regulate different genes and modulate various enzyme activities. In connection with the latter aspect, we show here that, in isolated rat hepatocytes, 1 microM 4-hydroxynonenal selectively activates protein kinase C-delta, involved in apoptosis of many cell types; it also induces very early activation of Jun N-terminal kinase, in parallel increasing activator protein-1 DNA-binding activity in a time-dependent manner and triggering apoptosis after only 120 min treatment. These phenomena are likely protein kinase C-delta-dependent, being significantly reduced or annulled by cell co-treatment with rottlerin, a selective inhibitor of protein kinase C-delta. We suggest that 4-hydroxynonenal may induce apoptosis through activation of protein kinase C-delta and of Jun N-terminal kinase, and consequent up-regulation of activator protein-1 DNA binding.

Resistance to oxidative stress by hyperplastic and neoplastic rat liver tissue monitored in terms of production of unpolar and medium polar carbonyls.

The susceptibility of rat liver tissue to oxidative stress during its neoplastic transformation was analyzed by both qualitative and quantitative measurements of the carbonyl products of lipid peroxidation. Diethylnitrosamine was used as initiating agent of hepatocarcinogenesis and lipid peroxidation levels were monitored in the homogenates from normal liver, hyperplastic nodules and tumour, incubated in the presence or in the absence of ascorbate or adenosine diphosphate-iron complex. While the basal levels of lipid peroxidation in the three experimental conditions were found to be quite similar, in the presence of the pro-oxidant stimulus a remarkable reduction in aldehyde production was shown not only by the hepatoma tissue but also by the preneoplastic nodules.

Down-modulation of nuclear localisation and pro-fibrogenic effect of 4-hydroxy-2,3-nonenal by thiol- and carbonyl-reagents.

Among the oxidative breakdown products of omega-6 unsaturated fatty acids, the aldehyde 4-hydroxy-2,3-nonenal (HNE) is receiving increasing attention for its potential pathophysiological implication, which at least partly lies on the demonstrated ability to modulate gene expression of a number of genes. Here we show that a marked down-modulation of HNE nuclear localisation in cells of a macrophage line (J774-A1) can be afforded by treatment with sulfydryl and carbonyl reagents without significantly interfering with cell viability. As regards the addition of thiol-group reagents to the cell suspension, N-ethylmaleimide (NEM) led to a sustained decrease of HNE nuclear localisation, while 4-(chloromercuri)-benzene-sulfonic acid (PCMBS) gave a similar but more transient effect. Hydroxylamine (HYD), a carbonyl-group reagent, was also able to inhibit HNE nuclear localisation. The actual efficacy of the inhibitors used was then tested on the HNE-induced stimulation of transforming growth factor beta1 (TGFbeta1) production by J774-A1 cells. Indeed, the thiol reagents NEM and PCMBS, both markedly down-modulating HNE nuclear localisation, were able to inhibit HNE-induced increase of TGFbeta1 protein synthesis. The carbonyl reagent HYD was less effective on this respect, producing strong but incomplete protection against HNE-induced TGFbeta1 increase. Taken together, the results indicate that sulfydryl groups are involved in the process of HNE cellular internalisation, while both sulfydryl and carbonyl groups are involved in the process of HNE nuclear translocation, and consequently in the modulation of gene expression by the aldehyde. Further, an actual demonstration is provided that HNE-induced effect on gene regulation can be efficiently counteracted by suitable interference with HNE biochemistry.

4-hydroxynonenal and transforming growth factor-beta1 expression in colon cancer.

In vivo studies on human colon adenocarcinoma showed decreased transforming growth factor-beta1 (TGF-beta1) antiproliferative cytokine content in tumour tissue related to malignancy progression, with a corresponding decrease in lipid peroxidation aldehydic end-product, 4-hydroxynonenal (HNE). The tumour mechanism to escape TGF-beta1-mediated growth inhibition may be due to an altered TGF-beta1 receptor system. Subsequent in vitro analyses showed a differential distribution of TGF-beta1 receptors depending on the human colon cancer cell line considered (CaCo-2 or HT-29): compared to HT-29 cells, CaCo-2 cells showed a decrease of the two main TGF-beta1 receptors, RI and RII. Notwithstanding their partial TGF-beta1 RI and RII deficiency, treatment of CaCo-2 cells with adequate doses of the cytokine (10 ng/ml) was able to induce apoptosis. Of note, co-treatment of these cells with 1 microM HNE increased the apoptotic effect. The constant low concentration of TGF-beta1 in the tumour mass may be related to the low content of antiproliferative HNE observed in colon cancer: the latter phenomenon, which reduces TGF-beta1 production in the tumour area, may represent a favourable condition for neoplastic progression. The enhancement of TGF-beta1-induced apoptosis by HNE in CaCo-2 cells supports this hypothesis. The different transcriptional components regulated by the distinct signaling pathways of these two molecules might be proposed; in particular, crosstalk between the MAPK and the Smad pathway could modulate and co-operate in the transcription of target genes involved in regulation of cell proliferation.

In vitro evidence for CCl4 metabolites covalently bound to lipoprotein micelles.

CCl4-induced impairment of the lipoprotein secretion pathway of intact rat hepatocytes was carried out using 14CCl4 to check the possibility of binding to lipoproteins by CCl4 metabolites. After separation of different cell suspension fractions by means of ultracentrifugation and chemical precipitation procedures, a significant amount of the radioisotope was found covalently bound to the lipid and protein components of low density lipoproteins. Suitable experiments demonstrated that the bound radioisotope was represented by CCl4 metabolites and not by unactivated CCl4.

Lipid oxidation products in cell signaling.

The recent research on the impact that oxidative changes of biolipids could have in pathophysiology serves to explain how free radical-driven reactions not only are considered as mere toxicologic events, but also modulators of cell activity and function. Oxidatively modified low-density lipoproteins are known to affect various cellular processes by modulating various molecular pathways and signaling nuclear transcription. Among the lipid oxidation products detectable in ox-LDLs, and also in the atherosclerotic plaques, 4-hydroxynonenal has been widely investigated. This aldehyde was shown to upregulate AP-1 transcription factor, signaling through the MAP kinase pathway, with eventual nuclear localization and induction of a series of genes. Further, oxidation products of cholesterol and cholesterol esters, in ox-LDL are of similar interest, especially in relation to the pathogenesis of fibrosclerotic lesions of the arterial wall.

Lipid peroxidation in human diseases: evidence of red cell oxidative stress after circulatory shock.

Erythrocytes obtained from human patients with circulatory shock of different aetiology consistently showed a strong increase in lipid peroxidation-derived aldehydes in comparison with red cells of normal adults. The highly toxic compound 4-hydroxynonenal has been recovered exclusively in the erythrocytes of the patients.

Oxidative stress in the development of human ischemic hepatitis during circulatory shock.

An increasing number of studies support the involvement of free radical-mediated oxidative reactions in the pathogenesis of tissue injury following ischemia reperfusion. In particular, a condition of oxidative stress is evident in patients with circulatory shock, a disease process often complicated by progressive organ failure sustained by inflammatory reactions. In all shock patients without signs of organ failure, a consistent increase of intermediate and final products of lipid peroxidation (lipid peroxides and aldehydes respectively) was observed. Impairment of the redox equilibrium in the tissues of these patients was confirmed by a significant reduction of glutathione and vitamin E hematic concentrations. Moreover, a selective increase of plasma aldehyde-protein adducts, actual proof of oxidative damage of macromolecules, is only present in the shock patients who, in addition, show hepatic cytolysis (ischemic hepatitis) as estimated by plasma levels of LDH5 isoenzyme. Aldehyde adducts well mark the progression of the disease towards multiple organ failure. Finally, the good statistical correlation between aldehyde-modified proteins and LDH5, as well as their distinct behaviour in control and ischemic hepatitis, support the involvement of oxidative damage in the expression and worsening of circulatory shock.

Oxidative damage in human liver transplantation.

The aim of this study was to evaluate oxygen-dependent hepatic reperfusion injury in humans following orthotopic liver transplantation. To this end, a number of blood indices of impaired tissue redox balance were monitored in 19 adult patients for 3 weeks after liver transplantation. Both red cell malonaldehyde and plasma lipid peroxides increased significantly soon after organ reperfusion. This finding was consistently accompanied by decreased plasma vitamin E and red cell total glutathione. A peak of oxidative stress, as measured by the parameters monitored, was evident within 24 h after reperfusion, together with a maximum expression of cytolysis, as measured by plasma alanine aminotransferase. The occurrence of redox imbalance after hepatic reperfusion was shown to be linearly related to irreversible cell damage. As regards the low plasma levels of the two antioxidants after reperfusion, only that of vitamin E appeared statistically related to oxidative stress. With the background of an increasing body of proof, mainly from animal models, the involvement of toxic oxygen metabolites in hepatic cytolysis following orthotopic liver transplantation appears likely. The statistical correlation among the markers of redox imbalance monitored indicates their combined use in further investigation.

Liver AP-1 activation due to carbon tetrachloride is potentiated by 1,2-dibromoethane but is inhibited by alpha-tocopherol or gadolinium chloride.

Experimental acute intoxication by prooxidant haloalkanes produces marked stimulation of hepatic lipid peroxidation and cytolysis, which is followed by tissue regeneration. Our aim was to clarify the role of oxidative imbalance in the activation of the redox-sensitive transcription factor, activator protein-1 (AP-1), which is involved in tissue repair. Rats were poisoned with a very low concentration of carbon tetrachloride, given alone or in combination with another hepatotoxin, 1,2-dibromoethane, to provide varying extents of oxidative damage. The level of AP-1-DNA binding was analyzed by electrophoretic mobility shift assay on liver extracts, obtained from rats killed 6 h after poisoning. Stimulation of lipid peroxidation and AP-1 upregulation were already established when the hepatic damage due to carbon tetrachloride +/-1,2-dibromoethane was beginning to appear. Rat supplementation with the antioxidant vitamin E completely inhibited AP-1 upregulation, thus supporting a causative role of membrane lipid oxidation in the observed modulation of the transcription factor. Moreover, activation of Kupffer cells appears to be a crucial step in the increased AP-1 binding to DNA, the latter being largely prevented by gadolinium chloride, a macrophage-specific inhibitor.

Oxidative damage and transforming growth factor beta 1 expression in pretumoral and tumoral lesions of human intestine.

The aim of this study was to evaluate a possible relationship between oxidative stress and transforming growth factor beta 1 (TGF beta 1) expression in human colon adenocarcinoma. Crohn's disease, an inflammatory pathology of the intestine often regarded to as precancerous, was also examined. Indices of impaired redox balance were monitored in blood and in bioptic samples from 10 adult patients with adenocarcinoma of the colon and from five patients with Crohn's disease. On tissue samples TGF beta 1 mRNA expression was also determined. Ten healthy adults provided normal reference values for plasma indices of oxidative stress, and normal tissue distant from the lesions was used for comparative analysis. Fluorescent adducts with plasma proteins of malonaldehyde (MDA) and 4-hydroxynonenal (HNE) were significantly lower than controls in the plasma from cancer patients and significantly higher in the plasma from Crohn's patients. In adenocarcinoma biopsies, susceptibility to lipid peroxidation processes and TGF beta 1 expression were below the relative control; in Crohn's disease, lipid peroxidation and cytokine expression were both above the relative control. The findings obtained suggest the existence of an association between oxidative damage and fibrogenic cytokine expression in the human intestine. Further studies are needed to conclusively prove the correlation between the two events.

Glutathione depletion induces apoptosis of rat hepatocytes through activation of protein kinase C novel isoforms and dependent increase in AP-1 nuclear binding.

Treatment of isolated rat hepatocytes with the glutathione depleting agents L-buthionine-S,R-sulfoximine or diethylmaleate reproduced various cellular conditions of glutathione depletion, from moderate to severe, similar to those occurring in a wide spectrum of human liver diseases. To evaluate molecular changes and possible cellular dysfunction and damage consequent to a pathophysiologic level of GSH depletion, the effects of this condition on protein kinase C (PKC) isoforms were investigated, since these are involved in the intracellular specific regulatory processes and are potentially sensitive to redox changes. Moreover, a moderate perturbation of cellular redox state was found to activate novel PKC isoforms, and a clear relationship was shown between novel kinase activation and nuclear binding of the redox-sensitive transcription factor, activator protein-1 (AP-1). Apoptotic death of a significant number of cells, confirmed in terms of internucleosomal DNA fragmentation was a possible effect of these molecular reactions, and was triggered by a condition of glutathione depletion usually detected in human liver diseases. Finally, the inhibition of novel PKC enzymatic activity in cells co-treated with rottlerin, a selective novel kinase inhibitor, prevented glutathione-dependent novel PKC up-regulation, markedly moderated AP-1 activation, and protected cells against apoptotic death. Taken together, these findings indicate the existence of an apoptotic pathway dependent on glutathione depletion, which occurs through the up-regulation of novel PKCs and AP-1.

Oxidative stress and cell signalling.

An increasing body of evidence from animal models, human specimens and cell lines points to reactive oxygen species as likely involved in the pathways, which convey both extracellular and intracellular signals to the nucleus, under a variety of pathophysiological conditions. Indeed, reactive oxygen species (ROS), in a concentration compatible with that detectable in human pathophysiology, appear able to modulate a number of kinases and phosphatases, redox sensitive transcription factors and genes. This type of cell signalling consistently implies the additional involvement of other bioactive molecules that stem from ROS reaction with cell membrane lipids. The present review aims to comprehensively report on the most recent knowledge about the potential role of ROS and oxidised lipids in signal transduction processes in the major events of cell and tissue pathophysiology. Among the lipid oxidation products of ROS-dependent reactivity, which appear as candidates for a signalling role, there are molecules generated by oxidation of cholesterol, polyunsaturated fatty acids and phospholipids, as well as lysophosphatidic acid and lysophospholipids, platelet activating factor-like lipids, isoprostanes, sphingolipids and ceramide.

Dehydroepiandrosterone pretreatment protects rats against the pro-oxidant and necrogenic effects of carbon tetrachloride.

A single intraperitoneal injection of dehydroepiandrosterone (3 beta-hydroxy-5-androsten-17-one, DHEA) 17 hr before carbon tetrachloride (CCl4) poisoning protects rats against liver injury induced by the haloalkane. In liver homogenates, both the increase in malondialdehyde production and the formation of fluorescent lipid peroxidation products are significantly reduced. Also, liver microsomes obtained from DHEA-pretreated rats incubated in vitro with CCl4 are less susceptible to lipid peroxidation than microsomes from normal animals. The release of liver enzymes into the blood is much reduced in DHEA-pretreated rats, confirming a cause-effect relationship between lipid peroxidation and hepatocyte death. Treatment with DHEA inhibits neither glucose-6-phosphate dehydrogenase activity in the cytosol, nor the microsomal mixed function oxidase system (cytochrome P450 content, aminopyrine demethylase and ethoxycoumarin de-ethylase activities). In animals treated with DHEA, the liver content of total glutathione and vitamin E is not modified. These results support the hypothesis that DHEA protects against CCl4-induced liver injury through its own antioxidant activity, rather than by interfering with the metabolism of the toxin or with the tissue level of primary antioxidants.

Carbon tetrachloride-induced inhibition of hepatocyte lipoprotein secretion: functional impairment of Golgi apparatus in the early phases of such injury.

Functional change of liver Golgi apparatus during carbon tetrachloride (CCl4) poisoning was demonstrated both in rat isolated hepatocytes and in the whole animal. The "in vitro" experimental model provided evidence of Golgi derangement early after giving the haloalkane. The "in vivo" analyses also showed that such an alteration involves both formative and secretory sides of the subcellular structure.

Paracetamol-stimulated lipid peroxidation in isolated rat and mouse hepatocytes.

Treatment of isolated hepatocytes from 3-methylcholanthrene induced rats with 1 mM paracetamol has been found to greatly decrease cellular reduced glutathione (GSH) content and to promote lipid peroxidation, evaluated as malonaldehyde (MDA) production and conjugated diene absorbance. A similar dosing of hepatocytes from phenobarbital-induced or normal rats is ineffective in that respect. On the other hand, the aspecific stimulation of the cytochrome P-450-mediated paracetamol activation due to acetone addition further increases GSH depletion as well as MDA production. Isolated hepatocytes with basal low GSH content are also more susceptible to paracetamol-induced lipid peroxidation, indicating that the rate of the drug metabolism and the cellular GSH content are critical factors in the determination of such peroxidative attack. In isolated mouse liver cells paracetamol does not require preliminary cytochrome P-450 induction to stimulate MDA formation, even at concentrations ineffective in rat cells. However, 5 mM paracetamol, despite a great depletion of cellular GSH content, does not promote MDA formation either in the rat or in the mouse hepatocytes. This effect may be due to the ability of paracetamol to scavenge lipid peroxides under defined conditions, as tested in various lipid peroxidizing systems. Membrane leakage of lactate dehydrogenase (LDH) is evident in paracetamol treated cells undergoing lipid peroxidation, but not when MDA formation is inhibited by high doses of the drug or by addition of anti-oxidants such as alpha-tocopherol and diphenylphenylenediamine (DPPD). Nevertheless in these conditions the covalent binding of activated paracetamol metabolites is not affected, suggesting that lipid peroxidation might play a role in the pathogenesis of liver damage following paracetamol overdose.

Biochemical evidence for chemical and/or topographic differences in the lipoperoxidative processes induced by CCl4 and iron.

Isolated rat hepatocytes, treated with CCl4 or ADP-Fe3+ complex show an enhanced lipid peroxidation and a decreased glucose 6-phosphatase activity. Lipid peroxidation is much more stimulated by ADP-Fe3+ or Fe3+ than by CCl4, when the metal and the haloalkane are used at a similar concentration. Increasing rates of lipid peroxidation in the different experimental conditions do not correlate with the degree of glucose 6-phosphatase inactivation, which is produced by CCl4 and not by a similar amount of ferric iron. In the case of iron, its intracellular concentration must be higher to give the enzyme inactivation exerted by CCl4. Higher intracellular levels of iron are reached when the metal is added to the cell suspension together with ADP. Under these conditions there is inactivation of glucose 6-phosphatase. Possible mechanisms accounting for a different enzyme sensitivity to iron and CCl4 are discussed.

CCl4-induced increase of hepatocyte free arachidonate level: pathogenesis and contribution to cell death.

A significant increase of the intracellular level of free arachidonic acid was observed in intact rat hepatocytes after poisoning with very low concentrations of CCl4 (0.129-0.172 mM), shown not to exert direct solvent effect. It seems likely that activation of phospholipase A2 (PLA2) is the mechanism mainly responsible for the rise of cytosolic arachidonate, since the latter is prevented by the PLA2 inhibitors indomethacin and mepacrine. The CCl4-induced delay of arachidonic acid incorporation within the cell membrane phospholipids partly contributes to its intracellular accumulation in the early phases of the poisoning. The lack of any significant protection by metabolic inhibitors (SKF 525A, metyrapone), antioxidant compounds (promethazine, diphenylphenylenediamine DPPD) or antioxidant procedures (rat pretreatment with vitamin E) leads to exclude an involvement of CCl4 biotransformation in the increase of intracellular free arachidonate. Finally, the PLA2 inhibitors employed in this study did not afford protection against the enzymic leakage of CCl4-treated hepatocytes.

In vivo potentiation of 1,2-dibromoethane hepatotoxicity by ethanol through inactivation of glutathione-s-transferase.

In the rat, a single ethanol (EtOH) pretreatment (2.5 g/kg b.w., per os) was able to strongly enhance the cytotoxicity of 1,2-dibromoethane (DBE)(87 mg/kg b.w., per os). The principal metabolic routes of DBE involve both oxidative and conjugative transformations. Microsomal cytochrome P450 content and dimethyl nitrosamine demethylase activity were not changed, while a significant loss of cytosolic total GSH-transferase was observed in rats killed 6 h after EtOH pretreatment. Pretreatment with methylpyrazole, an inhibitor of alcohol-dehydrogenase prevented the effects provoked by ethanol. The major EtOH metabolite, acetaldehyde. seemed thus to play a fundamental role in the mechanism responsible for the potentiation of DBE toxicity mediated by EtOH. To further support this hypothesis, disulfiram (75 mg/kg b.w.), an inhibitor of aldehyde dehydrogenase, was given i.p. to rats. When DBE was administered to disulfiram- and EtOH-pretreated rats, a marked increase of liver cytolysis was shown and cytosolic GSH-transferase activity was further inhibited if compared to that induced by EtOH treatment alone. The results are consistent with the hypothesis that EtOH given to rats increases DBE liver toxicity because its major metabolite, acetaldehyde, reduces the DBE conjugates to GSH transferase, with consequent shift of DBE metabolism to the oxidative route and accumulation of reactive oxidative intermediates no longer effectively conjugated with GSH.

Effects of ethanol metabolism on PKC activity in isolated rat hepatocytes.

Isolated rat hepatocytes were exposed to increasing concentrations of ethanol. During exposure of cells to ethanol a moderate but significant modification in the level of hepatic PKC c-isoforms has been observed. The ethanol-induced effect on liver protein kinase C was reversed by 4-methylpyrazole, an inhibitor of alcohol dehydrogenase, indicating that the conversion of ethanol to acetaldehyde may be involved in the enzyme inactivation. The involvement of the alcohol metabolite in PKC modifications was confirmed by the exposure of hepatocytes or partially purified liver enzyme to acetaldehyde concentrations of pathological interest.