Retinoic acid inhibits hepatic Jun N-terminal kinase-dependent signaling pathway in ethanol-fed rats

Retinoids in the Pathogenesis and Treatment of Liver Diseases

Vitamin A (VA), all-trans-retinol (ROL), and its analogs are collectively called retinoids. Acting through the retinoic acid receptors RARα, RARβ, and RARγ, all-trans-retinoic acid, an active metabolite of VA, is a potent regulator of numerous biological pathways, including embryonic and somatic cellular differentiation, immune functions, and energy metabolism. The liver is the primary organ for retinoid storage and metabolism in humans. For reasons that remain incompletely understood, a body of evidence shows that reductions in liver retinoids, aberrant retinoid metabolism, and reductions in RAR signaling are implicated in numerous diseases of the liver, including hepatocellular carcinoma, non-alcohol-associated fatty liver diseases, and alcohol-associated liver diseases. Conversely, restoration of retinoid signaling, pharmacological treatments with natural and synthetic retinoids, and newer agonists for specific RARs show promising benefits for treatment of a number of these liver diseases. Here we provide a comprehensive review of the literature demonstrating a role for retinoids in limiting the pathogenesis of these diseases and in the treatment of liver diseases.

A retinoic acid receptor β2 agonist protects against alcohol liver disease and modulates hepatic expression of canonical retinoid metabolism genes

Alcohol abuse reduces hepatic vitamin A (retinoids), reductions that are associated with progression of alcohol liver disease (ALD). Restoring hepatic retinoids through diet is contraindicated in ALD due to the negative effects of alcohol on retinoid metabolism. There are currently no drugs that can both mitigate alcohol-driven hepatic retinoid losses and progression of ALD. Using a mouse model of alcohol intake, we examined if an agonist for the retinoic acid (RA) receptor β2 (RARβ2), AC261066 (AC261) could prevent alcohol-driven hepatic retinoid losses and protect against ALD. Our results show that mice co-treated with AC261 and alcohol displayed mitigation of ALD, including reduced macro, and microvesicular steatosis, and liver damage. Alcohol intake led to increases in hepatic centrilobular levels of ALDH1A1, a rate-limiting enzyme in RA synthesis, and co-localization of ALDH1A1 with the alcohol-metabolizing enzyme CYP2E1, and 4-HNE, a marker of oxidative stress; expression of these targets was abrogated in mice co-treated with AC261 and alcohol. By RNA sequencing technology, we found that AC261 treatments opposed alcohol modulation of 68 transcripts involved in canonical retinoid metabolism. Alcohol modulation of these transcripts, including CES1D, CES1G, RBP1, RDH10, and CYP26A1, collectively favor hepatic retinoid hydrolysis and catabolism. However, despite this, co-administration of AC261 with alcohol did not mitigate alcohol-mediated depletions of hepatic retinoids, but did reduce alcohol-driven increases in serum retinol. Our data show that AC261 protected mice against ALD, even though AC261 did not prevent alcohol-mediated reductions in hepatic retinoids. These data warrant further studies of the anti-ALD properties of AC261.

Modeling PNPLA3-Associated NAFLD Using Human-Induced Pluripotent Stem Cells

BACKGROUND AND AIMS

NAFLD is a growing public health burden. However, the pathogenesis of NAFLD has not yet been fully elucidated, and the importance of genetic factors has only recently been appreciated. Genomic studies have revealed a strong association between NAFLD progression and the I148M variant in patatin-like phospholipase domain-containing protein 3 (PNPLA3). Nonetheless, very little is known about the mechanisms by which this gene and its variants can influence disease development. To investigate these mechanisms, we have developed an in vitro model that takes advantage of the unique properties of human-induced pluripotent stem cells (hiPSCs) and the CRISPR/CAS9 gene editing technology.

APPROACH AND RESULTS

We used isogenic hiPSC lines with either a knockout (PNPLA3KO ) of the PNPLA3 gene or with the I148M variant (PNPLA3I148M ) to model PNPLA3-associated NAFLD. The resulting hiPSCs were differentiated into hepatocytes, treated with either unsaturated or saturated free fatty acids to induce NAFLD-like phenotypes, and characterized by various functional, transcriptomic, and lipidomic assays. PNPLA3KO hepatocytes showed higher lipid accumulation as well as an altered pattern of response to lipid-induced stress. Interestingly, loss of PNPLA3 also caused a reduction in xenobiotic metabolism and predisposed PNPLA3KO cells to be more susceptible to ethanol-induced and methotrexate-induced toxicity. The PNPLA3I148M cells exhibited an intermediate phenotype between the wild-type and PNPLA3KO cells.

CONCLUSIONS

Together, these results indicate that the I148M variant induces a loss of function predisposing to steatosis and increased susceptibility to hepatotoxins.

Tomato lycopene prevention of alcoholic fatty liver disease and hepatocellular carcinoma development

Alcoholic liver disease (ALD) is a major cause of morbidity and mortality worldwide. The incidence of hepatocellular carcinoma (HCC) is increasing in the United States, and chronic, excessive alcohol consumption is responsible for 32%–45% of all the liver cancer cases in the United States. Avoidance of chronic or excessive alcohol intake is the best protection against alcohol-related liver injury; however, the social presence and addictive power of alcohol are strong. Induction of the cytochrome P450 2E1 (CYP2E1) enzyme by chronic and excessive alcohol intake is known to play a role in the pathogenesis of ALD. High intake of tomatoes, rich in the carotenoid lycopene, is associated with a decreased risk of chronic disease. The review will overview the prevention of ALD and HCC through dietary tomato rich in lycopene as an effective intervention strategy and the crucial role of CYP2E1 induction as a molecular target. The review also indicates a need for caution among individuals consuming both alcohol and high dose lycopene as a dietary supplement.

Host homeostatic responses to alcohol-induced cellular stress in animal models of alcoholic liver disease

Humans develop various clinical phenotypes of severe alcoholic liver disease, including alcoholic hepatitis and cirrhosis, generally after decades of heavy drinking. In such individuals, following each episode of drinking, their livers experience heightened intracellular and extracellular stresses that are closely associated with alcohol consumption and alcohol metabolism. This article focuses on the latest advances made in animal models on evolutionarily conserved homeostatic mechanisms for coping with and resolving these stress conditions. The mechanisms discussed include the stress-activated protein kinase JNK, energy regulator AMPK, autophagy and the inflammatory response. Over time, the host may respond variably to stress with protective mechanisms that are critical in determining an individual's vulnerability to developing severe alcoholic liver disease. A systematic review of these mechanisms and their temporal changes in animal models provides the basis for general conclusions, and raises questions for future studies. The relevance of these data to human conditions is also discussed.

Panax ginseng Meyer prevents radiation-induced liver injury via modulation of oxidative stress and apoptosis

Background

Radiotherapy is one of the most important modalities in cancer treatment; however, normal tissue damage is a serious concern. Drug development for the protection or reduction of normal tissue damage is therefore a clinical issue. Herein, we evaluated the protective properties of Panax ginseng Meyer and its corresponding mechanisms.

Methods

C56BL/6 mice were orally pretreated with P. ginseng water extract (PGE; 25 mg/kg, 50 mg/kg, or 100 mg/kg) or intraperitoneally injected melatonin (20 mg/kg) for 4 d consecutively, then exposed to 15-Gy X-ray radiation 1 h after the last administration. After 10 d of irradiation, the biological properties of hematoxicity, fat accumulation, histopathology, oxidative stress, antioxidant activity, pro-inflammatory cytokines, and apoptosis signals were examined in the hepatic tissue.

Results

The irradiation markedly induced myelosuppression as determined by hematological analysis of the peripheral blood. Steatohepatitis was induced by X-ray irradiations, whereas pretreatment with PGE significantly attenuated it. Oxidative stress was drastically increased, whereas antioxidant components were depleted by irradiation. Irradiation also notably increased serum liver enzymes and hepatic protein levels of pro-inflammatory cytokines. Those alterations were markedly normalized by pretreatment with PGE. The degree of irradiation-induced hepatic tissue apoptosis was also attenuated by pretreatment with PGE, which was evidenced by a terminal deoxynucleotidyl transferase 2′-deoxyuridine 5′-triphosphate nick-end labeling assay, western blotting, and gene expressions analysis, particularly of apoptotic molecules.

Conclusion

We suggest that PGE could be applicable for use against radiation-induced liver injury, and its corresponding mechanisms involve the modulation of oxidative stress, inflammatory reactions, and apoptosis.

Deletion of tumor progression locus 2 attenuates alcohol-induced hepatic inflammation

BACKGROUND

The pathogenesis of alcoholic liver disease (ALD) involves the interaction of several inflammatory signaling pathways. Tumor progression locus 2 (TPL2), also known as Cancer Osaka Thyroid (COT) and MAP3K8, is a serine-threonine kinase that functions as a critical regulator of inflammatory pathways by up-regulating production of inflammatory cytokines. The present study aims to fill the gap in knowledge regarding the involvement of TPL2 in the mechanism of alcohol-induced hepatic inflammation.

METHODS

Male TPL2(-/-) knockout (TPL2KO) mice and TPL2(+/+) wild-type (WT) mice were group pair-fed with Lieber-DeCarli liquid ethanol diet (EtOH diet, 27% energy from EtOH) or control diet (ctrl diet) for 4 weeks. Both histological and molecular biomarkers involved in the induction of hepatic inflammation by alcohol consumption were examined.

RESULTS

Consumption of the EtOH diet in WT mice lead to a significant induction of TPL2 mRNA expression as compared with WT mice fed ctrl diet. A significant induction in inflammatory foci and steatosis was also observed in WT mice fed EtOH diet. The deletion of TPL2 significantly reduced inflammatory foci in the liver of mice consuming both ctrl and EtOH diets as compared to their respective WT controls. This reduction was associated with suppression of hepatic inflammatory gene expression of interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), and interleukin-1β (IL-1β) and macrophage marker F4/80. In addition, histological analysis of livers revealed that TPL2 deletion resulted in reduced steatosis in both ctrl (significant) and EtOH (non-significant) diet-fed mice as compared to their respective WT controls.

CONCLUSIONS

The demonstration that TPL2 deletion attenuates alcohol-induced hepatic inflammation provides evidence of a novel role for TPL2 in the pathogenesis of ALD.

Dietary tomato powder inhibits alcohol-induced hepatic injury by suppressing cytochrome p450 2E1 induction in rodent models

Chronic and excessive alcohol consumption leads to the development of alcoholic liver disease (ALD) and greatly increases the risk of liver cancer. Induction of the cytochrome p450 2E1 (CYP2E1) enzyme by chronic and excessive alcohol intake is known to play a role in the pathogenesis of ALD. High intake of tomatoes, rich in the carotenoid lycopene, is associated with a decreased risk of chronic disease. We investigated the effects of whole tomato (tomato powder, TP), partial tomato (tomato extract, TE), and purified lycopene (LYC) against ALD development in rats. Of the three supplements, only TP reduced the severity of alcohol-induced steatosis, hepatic inflammatory foci, and CYP2E1 protein levels. TE had no effect on these outcomes and LYC greatly increased inflammatory foci in alcohol-fed rats. To further support the protective effect of TP against ALD, TP was supplemented in a carcinogen (diethylnitrosamine, DEN)-initiated alcohol-promoted mouse model. In addition to reduced steatosis and inflammatory foci, TP abolished the presence of preneoplastic foci of altered hepatocytes in DEN-injected mice fed alcohol. These reductions were associated with decreased hepatic CYP2E1 protein levels, restored levels of peroxisome proliferator-activated receptor-α and downstream gene expression, decreased inflammatory gene expression, and reduced endoplasmic reticulum stress markers. These data provide strong evidence for TP as an effective whole food prevention strategy against ALD.

Ethanol Mediates Cell Cycle Arrest and Apoptosis in SK-N-SH Neuroblastoma Cells

Background:

The mechanisms of cell or organ damage by chronic alcohol consumption are still poorly understood. The present study aimed to investigate the role of the mitogen-activated protein kinases during ethanol-induced damage to SK-N-SH neuroblastoma cells.

Methods:

Cells were treated with ethanol and subsequently analyzed for cell morphology, viability, and DNA fragmentation. Immunoblot analysis was performed to assess various proteins levels associated with cell cycle arrest and apoptosis after ethanol exposure.

Results:

Ethanol induced time- and dose-dependent cell death in SK-N-SH cells and increased c-Jun N-terminal protein kinase (JNK) activity in a time- and concentration dependent manner. In contrast, p38 kinase activity increased transiently. After treatment with JNK or p38 kinase inhibitors, ethanol-induced cell death significantly reduced. Ethanol-induced cell death was accompanied by increased cytochrome c release and caspase 3 activity observed at 12 h. In contrast, the level of anti-apoptotic Bcl-2 protein did not change. Ethanol also increased the phosphorylation of p53 and p53 activation was followed by an increase in the p21 tumor suppressor protein accompanied by a gradual decrease in phospho-Rb protein.

Conclusion:

Our results suggest that ethanol mediates apoptosis of neuroblastoma cells by stimulating p53-related cell cycle arrest mediated through activation of the JNK-related pathway.

Carotenoids and alcoholic liver disease

Chronic and excessive consumption of alcohol leads to the development of alcoholic liver disease. The depletion of vitamin A is a well-known consequence of alcohol consumption, and may be associated with the observed alcohol-induced hepatic injury. The provitamin A carotenoid β-carotene has been demonstrated to increase alcohol-induced hepatic injury when given in high doses, while low dose supplementation provides protection against hepatic injury. However, it is unknown if the hepatoprotective effects of low dose β-carotene are due to the protective actions of β-carotene itself or if the alterations are due to restored vitamin A levels. Future studies are needed to provide further insight into the specific mechanisms by which β-carotene exerts its protective effect. Further, supplementation studies utilizing high doses of β-carotene in the presence of alcohol must be done with caution.

Low-dose ATRA supplementation abolishes PRM formation in rat liver and ameliorates ethanol-induced liver injury

The effects of all-trans-retinoic acid (ATRA) in low doses supplementation on concentrations of polar retinoid metabolites (PRM) and retinoids in the ethanol-fed rat liver, and on hepatocyte injury were investigated. The rat model of alcoholic liver disease (ALD) was induced by intragastric infusion of ethanol, and then the rats were administrated with ATRA in two different doses (150 microg/kg body weight and 1.5 mg/kg body weight) for 4 weeks. Concentrations of retinoids in rat liver and plasma were determined by using HPLC. Liver tissues pathologic changes were observed under the light microscopy and electron microscopy. The serum transaminases concentrations were measured. The results showed that the HPLC analysis of retinoids revealed that retinoids (vitamin A, RA, retinyl palmitate) concentrations in ethanol-fed rat liver and RA concentration in ethanol-fed rat plasma were markedly diminished (P<0.01) after ethanol feeding for 12 weeks. Furthermore, obvious peaks of PRM were formed in livers of ethanol-fed rats. ATRA 150 microg/kg supplementation in ethanol-fed rats for 4 weeks raised RA concentration in both liver and plasma, and also raised vitamin A concentration in liver to control levels, partially restored retinyl palmitate concentration (P<0.05) in liver. ATRA 1.5 mg/kg supplementation raised not only RA concentrations in liver and plasma but also retinyl palmitate concentrations in liver. However, the vitamin A concentration in liver of ATRA-supplemented rats (1.5 mg/kg) was higher than that of controls (P<0.05). The histologic observation of liver tissues indicated that ATRA treatment notably alleviated hepatocellular swelling, steatosis, the swelling of mitochondria and proliferation of smooth endoplasmic reticulum (SER). ATRA treatment greatly decreased levels of serum transaminases as compared with the only ethanol-fed group (P<0.05). It was concluded that low-dose ATRA treatment could restore retinoids concentrations and abolish the PRM formation in liver of ALD rats, and then ameliorate the injury of liver cells.

Cytochrome P450 2E1 inhibition prevents hepatic carcinogenesis induced by diethylnitrosamine in alcohol-fed rats

Chronic alcohol ingestion increases hepatic cytochrome P450 2E1 (CYP2E1), which is associated with hepatocarcinogenesis. We investigated whether treatment with chlormethiazole (CMZ), a CYP2E1 inhibitor, protects against alcohol-associated hepatic carcinogenesis in rats. Rats were fed either an ethanol liquid diet or a non-ethanol liquid diet, with or without CMZ for one and ten months. A single intraperitoneal injection of diethylnitrosamine (DEN, 20 mg/kg) was given to initiate hepatic carcinogenesis. CYP2E1 expression, inflammatory proteins, cell proliferation, protein-bound 4-HNE, etheno-DNA adducts, 8-hydroxy-2'-deoxyguanosine (8-OHdG), retinoid concentrations, and hepatic carcinogenesis were examined. Ethanol feeding for 1 month with DEN resulted in significantly increased hepatic CYP2E1 levels and increased nuclear accumulation of NF-κB protein and TNF-α expression, which were associated with increased cyclin D1 expression and p-GST positive altered hepatic foci. All of these changes induced by ethanol feeding were significantly inhibited by the one month CMZ treatment. At 10-months of treatment, hepatocellular adenomas were detected in ethanol-fed rats only, but neither in control rats nor in animals receiving ethanol and CMZ. The 8-OHdG formation was found to be significantly increased in ethanol fed animals and normalized with CMZ treatment. In addition, alcohol-reduced hepatic retinol and retinoic acid concentrations were restored by CMZ treatment to normal levels in the rats at 10 months of treatment. These data demonstrate that the inhibition of ethanol-induced CYP2E1 as a key pathogenic factor can counteract the tumor-promoting action of ethanol by decreasing TNF-α expression, NF-κB activation, and oxidative DNA damage as well as restoring normal hepatic levels of retinoic acid in DEN-treated rats.

The adverse effects of alcohol on vitamin A metabolism

The objective of this review is to explore the relationship between alcohol and the metabolism of the essential micronutrient, vitamin A; as well as the impact this interaction has on alcohol-induced disease in adults. Depleted hepatic vitamin A content has been reported in human alcoholics, an observation that has been confirmed in animal models of chronic alcohol consumption. Indeed, alcohol consumption has been associated with declines in hepatic levels of retinol (vitamin A), as well as retinyl ester and retinoic acid; collectively referred to as retinoids. Through the use of animal models, the complex interplay between alcohol metabolism and vitamin A homeostasis has been studied; the reviewed research supports the notion that chronic alcohol consumption precipitates a decline in hepatic retinoid levels through increased breakdown, as well as increased export to extra-hepatic tissues. While the precise biochemical mechanisms governing alcohol's effect remain to be elucidated, its profound effect on hepatic retinoid status is irrefutable. In addition to a review of the literature related to studies on tissue retinoid levels and the metabolic interactions between alcohol and retinoids, the significance of altered hepatic retinoid metabolism in the context of alcoholic liver disease is also considered.

Hepatic metabolism of retinoids and disease associations

The liver is the most important tissue site in the body for uptake of postprandial retinoid, as well as for retinoid storage. Within the liver, both hepatocytes and hepatic stellate cells (HSCs) are importantly involved in retinoid metabolism. Hepatocytes play an indispensable role in uptake and processing of dietary retinoid into the liver, and in synthesis and secretion of retinol-binding protein (RBP), which is required for mobilizing hepatic retinoid stores. HSCs are the central cellular site for retinoid storage in the healthy animal, accounting for as much as 50-60% of the total retinoid present in the entire body. The liver is also an important target organ for retinoid actions. Retinoic acid is synthesized in the liver and can interact with retinoid receptors which control expression of a large number of genes involved in hepatic processes. Altered retinoid metabolism and the accompanying dysregulation of retinoid signaling in the liver contribute to hepatic disease. This is related to HSCs, which contribute significantly to the development of hepatic disease when they undergo a process of cellular activation. HSC activation results in the loss of HSC retinoid stores and changes in extracellular matrix deposition leading to the onset of liver fibrosis. An association between hepatic disease progression and decreased hepatic retinoid storage has been demonstrated. In this review article, we summarize the essential role of the liver in retinoid metabolism and consider briefly associations between hepatic retinoid metabolism and disease. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Animal models in carotenoids research and lung cancer prevention

Numerous epidemiological studies have consistently demonstrated that individuals who eat more fruits and vegetables (which are rich in carotenoids) and who have higher serum β-carotene levels have a lower risk of cancer, especially lung cancer. However, two human intervention trials conducted in Finland and in the United States have reported contrasting results with high doses of β-carotene supplementation increasing the risk of lung cancer among smokers. The failure of these trials to demonstrate actual efficacy has resulted in the initiation of animal studies to reproduce the findings of these two studies and to elucidate the mechanisms responsible for the harmful or protective effects of carotenoids in lung carcinogenesis. Although these studies have been limited by a lack of animal models that appropriately represent human lung cancer induced by cigarette smoke, ferrets and A/J mice are currently the most widely used models for these types of studies. There are several proposed mechanisms for the protective effects of carotenoids on cigarette smoke-induced lung carcinogenesis, and these include antioxidant/prooxidant effects, modulation of retinoic acid signaling pathway and metabolism, induction of cytochrome P450, and molecular signaling involved in cell proliferation and/or apoptosis. The technical challenges associated with animal models include strain-specific and diet-specific effects, differences in the absorption and distribution of carotenoids, and differences in the interactions of carotenoids with other antioxidants. Despite the problems associated with extrapolating from animal models to humans, the understanding and development of various animal models may provide useful information regarding the protective effects of carotenoids against lung carcinogenesis.

Depletion of cytosolic or mitochondrial thioredoxin increases CYP2E1-induced oxidative stress via an ASK-1-JNK1 pathway in HepG2 cells

Thioredoxin is an important reducing molecule in biological systems. Increasing CYP2E1 activity induces oxidative stress and cell toxicity. However, whether thioredoxin protects cells against CYP2E1-induced oxidative stress and toxicity is unknown. SiRNA were used to knockdown either cytosolic (TRX-1) or mitochondrial thioredoxin (TRX-2) in HepG2 cells expressing CYP2E1 (E47 cells) or without expressing CYP2E1 (C34 cells). Cell viability decreased 40-60% in E47 but not C34 cells with 80-90% knockdown of either TRX-1 or TRX-2. Depletion of either thioredoxin also potentiated the toxicity produced either by a glutathione synthesis inhibitor or by TNFα in E47 cells. Generation of reactive oxygen species and 4-HNE protein adducts increased in E47 but not C34 cells with either thioredoxin knockdown. GSH was decreased and adding GSH completely blocked E47 cell death induced by either thioredoxin knockdown. Lowering TRX-1 or TRX-2 in E47 cells caused an early activation of ASK-1, followed by phosphorylation of JNK1 after 48 h of siRNA treatment. A JNK inhibitor caused a partial recovery of E47 cell viability after thioredoxin knockdown. In conclusion, knockdown of TRX-1 or TRX-2 sensitizes cells to CYP2E1-induced oxidant stress partially via ASK-1 and JNK1 signaling pathways. Both TRX-1 and TRX-2 are important for defense against CYP2E1-induced oxidative stress.

Long-term ethanol consumption promotes hepatic tumorigenesis but impairs normal hepatocyte proliferation in rats

Chronic and excessive alcohol consumption has been related to an increased risk of several cancers, including that of the liver; however, studies in animal models have yet to conclusively determine whether ethanol acts as a tumor promoter in hepatic tumorigenesis. We examined whether prolonged alcohol consumption could act as a hepatic tumor promoter after initiation by diethylnitrosamine (DEN) in a rat model. Male Sprague-Dawley rats were injected with 20 mg DEN/kg body weight 1 wk before introduction of either an ethanol liquid diet or an isoenergic control liquid diet. Hepatic pathological lesions, hepatocyte proliferation, apoptosis, PPARα and PPARγ, and plasma insulin-like growth factor 1 (IGF-1) levels were assessed after 6 and 10 mo. Mean body and liver weights, plasma IGF-1 concentration, hepatic expressions of proliferating cellular nuclear antigen and Ki-67, and cyclin D1 in ethanol-fed rats were all significantly lower after 10 mo of treatment compared with control rats. In addition, levels of hepatic PPARγ protein, not PPARα, were significantly higher in the ethanol-fed rats after prolonged treatment. Although ethanol feeding also resulted in significantly fewer altered hepatic foci, hepatocellular adenoma was detected in ethanol-fed rats at 10 mo, but not in control rats given the same dose of DEN. Together, these results indicate that chronic, excessive ethanol consumption impairs normal hepatocyte proliferation, which is associated with reduced IGF-1 levels, but promotes hepatic carcinogenesis.

Dietary lycopene and tomato extract supplementations inhibit nonalcoholic steatohepatitis-promoted hepatocarcinogenesis in rats

Epidemiological and experimental studies provide supportive evidence that lycopene (LY), a major carotenoid from tomatoes and tomato products, may act as a chemopreventive agent against certain types of cancers. We recently showed that high-fat diet (HFD)-induced nonalcoholic steatohepatitis (NASH) promoted diethylnitrosamine (DEN)-initiated hepatocarcinogenesis in a rat model. Using this model, we investigated the efficacy of an equivalent dosage of dietary LY from either a pure compound or a tomato extract (TE) against NASH-promoted hepatocarcinogenesis. Six groups of rats were injected with DEN and then fed either Lieber-DeCarli control diet or HFD with or without LY or TE for 6 weeks. Results showed that both LY and TE supplementations significantly decreased the number of altered hepatic foci expressing the placental form of glutathione S-transferase in the livers of HFD-fed rats. This was associated with significantly lower proliferating cell nuclear antigen positive hepatocytes and cyclinD1 protein, as well as decreased activation of extracellular signal-regulated kinase and nuclear NF-kappaB. Although both LY and TE supplementations reduced HFD-induced lipid peroxidation in the livers, we observed significantly decreased cytochrome P450 2E1, inflammatory foci and mRNA expression of proinflammatory cytokines (TNF-alpha, IL-1beta and IL-12) in the HFD+TE fed group but increased nuclear NF-E2-related factor-2 and heme oxygenase-1 proteins in the HFD+LY fed group, relative to HFD feeding alone. These data indicate that LY and TE can inhibit NASH-promoted hepatocarcinogenesis mainly as a result of reduced oxidative stress, which could be fulfilled through different mechanisms.

Vitamin E supplementation does not prevent ethanol-reduced hepatic retinoic acid levels in rats

Chronic, excessive ethanol intake can increase retinoic acid (RA) catabolism by inducing cytochrome P450 2E1 (CYP2E1). Vitamin E (VE) is an antioxidant implicated in CYP2E1 inhibition. In the current study, we hypothesized that VE supplementation inhibits CYP2E1 and decreases RA catabolism, thereby preventing ethanol-induced hepatocyte hyperproliferation. For 1 month, 4 groups of Sprague-Dawley rats were fed a Lieber-DeCarli liquid ethanol (36% of the total energy) diet as follows: either ethanol alone (Alc group) or ethanol in combination with 0.1 mg/kg body weight of all-trans-RA (Alc + RA group), 2 mg/kg body weight of VE (Alc + VE group), or both together (Alc + RA + VE group). Control rats were pair-fed a liquid diet with an isocaloric amount of maltodextrin instead of ethanol. The ethanol-fed groups had 3-fold higher hepatic CYP2E1 levels, 50% lower hepatic RA levels, and significantly increased hepatocyte proliferation when compared with the controls. The ethanol-fed rats given VE had more than 4-fold higher hepatic VE concentrations than the ethanol-fed rats without VE, but this did not prevent ethanol induction of CYP2E1, lower hepatic retinoid levels, or hepatocellular hyperproliferation. Furthermore, VE supplementation could not prevent RA catabolism in liver microsomal fractions of the ethanol-fed rats in vitro. These results show that VE supplementation can neither inhibit ethanol-induced changes in RA catabolism nor prevent ethanol-induced hepatocyte hyperproliferation in the rat liver.

Moderate alcohol consumption aggravates high-fat diet induced steatohepatitis in rats

BACKGROUND

Nonalcoholic steatohepatitis (NASH) develops in the absence of chronic and excessive alcohol consumption. However, it remains unknown whether moderate alcohol consumption aggravates liver inflammation in pre-existing NASH condition.

METHODS

Sprague-Dawley rats were first fed ad libitum with Lieber-DeCarli high-fat diet (71% energy from fat) for 6 weeks to induce NASH, as demonstrated previously. Afterwards, these rats were continuously fed with high-fat diet (HFD, 55% total energy from fat) or high fat plus alcohol diet (HFA, 55% energy from fat and 16% energy from alcohol) for an additional 4 weeks. Pathological lesions including fat accumulation and inflammatory foci in liver were examined and graded. Lipid peroxidation and apoptotic hepatocytes in the liver were assessed. The mRNA expressions of tumor necrosis factor-alpha (TNFalpha) and TNF receptor 1 (TNF-R1), Fas death receptor (Fas) and Fas ligant (FasL), IL-1beta and IL-12 were determined by real-time PCR. Protein levels of total and cleaved caspase-3, CYP2E1, Bax, and Bcl-2 were measured by western blotting.

RESULTS

The number of hepatic inflammatory foci and apoptotic hepatocytes were significantly increased in rats fed with HFA as compared with those in HFD-fed rats. The aggravated inflammatory response and cellular apoptosis mediated by HFA were associated with elevated mRNA expression of Fas/FasL and cleaved caspase-3 protein. Although no significant differences were observed between HFD and HFA groups, the levels of lipid peroxidation, Bax and Bcl-2 protein concentration, and mRNA levels of other inflammatory cytokines were significantly higher in these 2 groups than those in the control group.

CONCLUSIONS

These data suggest that even moderate alcohol consumption can cause more hepatic inflammation and cellular apoptosis in a pre-existing NASH condition.

Chronic ethanol feeding potentiates Fas Jo2-induced hepatotoxicity: role of CYP2E1 and TNF-alpha and activation of JNK and P38 MAP kinase

We have previously shown that treatment of mice with pyrazole or acute ethanol potentiated Fas agonistic Jo2 antibody-induced liver injury by a mechanism involving induction of CYP2E1 and elevated oxidative stress. The current study evaluated whether chronic alcohol feeding potentiates Fas-induced liver injury and whether CYP2E1 plays a role in any enhanced hepatotoxicity. Wild-type and CYP2E1 knockout mice were fed ethanol or isocaloric dextrose for 4 weeks followed by a single treatment with either saline or Jo2. Mice were killed 8 h after the Jo2 challenge. There were three- to five fold increases in transaminases and more extensive eosinophilic necrosis, hemorrhage, and infiltration of inflammatory cells in the central zone of the hepatic lobule in the ethanol-fed mice treated with Jo2 compared to the dextrose/Jo2- or ethanol/saline-treated mice. Liver injury was blunted in ethanol-fed CYP2E1 knockout mice treated with Jo2. The chronic ethanol feeding produced steatosis, elevation of CYP2E1, and oxidative stress in wild-type but not CYP2E1 knockout mice. These changes in wild-type mice fed ethanol were similar after saline or Jo2 treatment. The Jo2 treatment produced activation of JNK and P38 MAP kinase, increased activity of caspase-8 and -3, and lowered hepatic GSH levels in both the dextrose- and the alcohol-fed mice. JNK was activated at early times after Jo2 treatment in the ethanol-fed mice. Serum TNF-alpha levels were strikingly elevated in the wild-type ethanol/Jo2 group, which showed liver injury, compared to all the other groups, which did not show liver injury. Inhibition of JNK or P38 MAPK partially, but not completely, prevented the elevated liver injury in the wild-type ethanol/Jo2 mice. These results show that chronic ethanol feeding enhances Fas-induced liver injury by a mechanism associated with induction of CYP2E1, elevated serum TNF-alpha levels, and activation of MAPK.

Nonalcoholic steatohepatitis induced by a high-fat diet promotes diethylnitrosamine-initiated early hepatocarcinogenesis in rats

It has been suggested that patients with nonalcoholic steatohepatitis (NASH) may have high risk for liver cancer. However, it is unknown whether high-fat diet (HFD) induced NASH promotes hepatocarcinogenesis. In this study, Sprague-Dawley rats were injected with a low dose of hepatic carcinogen diethylnitrosamine (DEN) and then fed either Lieber-DeCarli control diet (CD) or HFD for 6 weeks. Liver histology and the hepatic placental form of glutathione S-transferase (P-GST) positive foci were examined. Expression levels of proliferating cell nuclear antigen (PCNA), cyclinD1, phosphorylated mitogen-activated protein kinase (MAPK) including extracellular signal-regulated kinase (ERK) and p38, as well as tumor necrosis factor-alpha (TNF-alpha), and nuclear factor-kappaB (NF-kappaB) were measured in the liver. Induction of lipid peroxidation end products (malondialdehyde plus 4-hydroxynonenal) in liver and apoptotic hepatocytes were also assessed. Results showed that HFD-fed rats developed significantly higher incidence and multiplicity of P-GST positive foci along with more fat accumulation, infiltration of inflammatory cells and higher lipid peroxidation in the liver, when compared with rats fed the CD. This high prevalence of hepatic lesions in the liver was accompanied by greater PCNA expression and cyclinD1 protein concentration but little change in hepatocyte apoptosis. HFD feeding elevated hepatic phosphorylated ERK but reduced phosphorylated p38 when compared with the CD feeding. In addition, a significantly higher expression of TNF-alpha mRNA and nuclear NF-kappaB p65 protein were observed in HFD group than in CD group. These data clearly demonstrate that NASH induced by HFD promoted DEN-initiated early hepatocarcinogenesis, which was associated with elevated TNF-alpha/NF-kappaB signaling and MAPK related hepatocyte proliferation.

Increased apoptosis in high-fat diet-induced nonalcoholic steatohepatitis in rats is associated with c-Jun NH2-terminal kinase activation and elevated proapoptotic Bax

Hepatocyte apoptosis in addition to oxidative stress could be a key component in the pathogenesis of nonalcoholic steatohepatitis (NASH). However, the underlying mechanisms of hepatocellular apoptotic response associated with oxidative stress have not been investigated in high-fat diet (HFD)-induced NASH models. In this study, Sprague-Dawley rats were fed either a Lieber-DeCarli control diet (CD; 35% energy from fat) or a HFD (71% energy from fat) for 6 wk. Pathologic lesions, lipid peroxidation products, and apoptotic hepatocytes in the liver were examined. The expressions of hepatic tumor necrosis factor-alpha (TNFalpha) and protein concentrations of cleaved caspase-3, cytochrome p4502E1 (CYP2E1), phosphorylated c-Jun NH(2)-terminal kinase (JNK), Bax, Bcl-2, and Bcl-xl were measured. Results showed that the key histological features of NASH, including steatosis, inflammatory cell infiltration, and ballooning degeneration of hepatocytes, were induced by HFD feeding, with increased hepatic TNFalpha mRNA expression. HFD-fed rats had elevated lipid peroxidation products and CYP2E1 protein in the liver. The apoptotic hepatocytes were significantly greater in livers of rats fed HFD than in those fed CD, and these were associated with a higher level of cleaved caspase-3. In addition, HFD feeding increased both hepatic phosphorylated JNK and pro-apoptotic Bax but did not affect anti-apoptotic Bcl-2 and Bcl-xl compared with CD feeding. These data indicate that the increased oxidative stress and its associated JNK activation as well as an imbalance of pro- and anti-apoptotic proteins in the Bcl-2 family all contribute to high hepatocyte apoptosis that may play an important role in the pathogenesis of NASH in this model.

Dosage effects of ginkgolide B on ethanol-induced cell death in human hepatoma G2 cells

Ginkgolide B is a major active component of Ginkgo biloba extracts, which has been shown to confer anticancer effects by inducing apoptosis or inhibiting oxidative stress generation. Ethanol induces a wide range of cellular toxicities, many of which have been linked to free radical generation. To further elucidate the cellular effects of ginkgolide B, we examined the dose-response effect of ginkgolide B on ethanol-induced toxicity in human Hep G2 cells. TUNEL and MTT assays revealed that ethanol (50-400 mM) induced apoptotic cell death in human Hep G2 cells, and that this effect was inhibited by low (5-25 microM) doses of ginkgolide B, but enhanced by high (50-100 microM) doses of ginkgolide B. Additional experiments revealed that ethanol treatment directly increased intracellular oxidative stress; this effect was enhanced by high doses of ginkgolide B but decreased following treatment with low concentrations of ginkgolide B. The dose-response effects of ginkgolide B on reactive oxygen species (ROS) generation were directly correlated with cell apoptotic biochemical changes including c-Jun N-terminal kinase (JNK) activation, caspase-3 activation, and DNA fragmentation. These results indicate that treatment dosage may determine the effect of ginkgolide B on ethanol-induced ROS generation and cell apoptosis, and support the notion that an appropriate dosage of ginkgolide B may aid in decreasing the toxic effects of ethanol.

Dosage effects of curcumin on cell death types in a human osteoblast cell line

Curcumin, the yellow pigment of Curcuma longa, is known to have antioxidant and anti-inflammatory properties, as well as their ability to either induce or prevent cell apoptosis. However, the precise molecular mechanisms of these effects are unknown. Here, we demonstrate that curcumin can induce apoptotic changes, including JNK activation, caspase-3 activation, and cleavage of PARP and PAK2, at treatment concentrations lower than 25 microM in human osteoblast cells. In contrast, treatment with 50-200 microM of curcumin does not induce apoptosis, but rather triggers necrotic cell death in human osteoblasts. Using the cell permeable dye 2',7'-dichlorofluorescin diacetate (DCF-DA) as an indicator of reactive oxygen species (ROS) generation, we found that while treatment with 12.5-25 microM curcumin directly increased intracellular oxidative stress, 50-200 microM curcumin had far less effect. Pretreatment of cells with N-acetyl cysteine or alpha-tocopherol, two well known ROS scavengers, attenuated the intracellular ROS levels increases and converted the apoptosis to necrosis induced by 12.5-25 microM curcumin. Moreover, we observed a dose-dependent decrease in intracellular ATP levels after treatment of osteoblast cells with curcumin and pretreatment of cells with antimycin or 2-deoxyglucose to cause ATP depletion significantly converted 12.5-25 microM curcumin-induced apoptosis to necrosis, indicating that ATP (a known mediator of apoptotic versus necrotic death) is most likely involved in the switching mechanism. Overall, our results signify that curcumin dosage treatment determines the possible effect on ROS generation, intracellular ATP levels, and cell apoptosis or necrosis in osteoblast cells.

Ethanol reduces p38 kinase activation and cyclin D1 protein expression after partial hepatectomy in rats

BACKGROUND/AIMS

Chronic ethanol consumption inhibits liver regeneration. We examined the effects of chronic ethanol consumption on two mitogen-activated protein kinases in relation to induction of cell cycle proteins after partial hepatectomy (PH).

METHODS

Male Wistar rats were ethanol-fed (EF) or pair-fed (PF) for 16 weeks before PH. Hepatic activation of extracellular signal regulated kinase (ERK)1/2, p38 kinase and expression of cyclinD1, cyclin-dependent kinase-4 (cdk4) and proliferating cell nuclear antigen (PCNA) were studied.

RESULTS

In PF rats, PH-induced p38 activation was evident at 2h and was maximal at 12h. There was a close temporal relationship between p38 activation, cyclin D1 and PCNA expression. Alcohol exposure reduced p38 activation, cyclin D1 and PCNA, each by approximately 50%. ERK1/2 activation occurred during the first 2h post-PH in both EF and PF rats, and there was no later increase in PF rats. In vivo inhibition of p38 suppressed PCNA expression whereas the effect of ERK1/2 inhibition was inconsistent.

CONCLUSIONS

p38 kinase activation is linked temporally with cyclin D1 expression after PH and appears to exert cell cycle control in the adult liver. p38 signaling also appears to be a target for the inhibitory effect of chronic alcohol on liver regeneration.

Combined antioxidant (beta-carotene, alpha-tocopherol and ascorbic acid) supplementation increases the levels of lung retinoic acid and inhibits the activation of mitogen-activated protein kinase in the ferret lung cancer model

Interactions among beta-carotene (BC), alpha-tocopherol (AT) and ascorbic acid (AA) led to the hypothesis that using a combination of these antioxidants could be more beneficial than using a single antioxidant alone, particularly against smoke-related lung cancer. In this investigation, we have conducted an animal study to determine whether combined BC, AT and AA supplementation (AOX) protects against 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)-induced lung carcinogenesis in smoke-exposed (SM) ferrets. Ferrets were treated for 6 months in the following four groups: (i) control, (ii) SM + NNK, (iii) AOX and (iv) SM + NNK + AOX. Results showed that the combined AOX supplementation (i) prevented the SM + NNK-decreased lung concentrations of retinoic acid (RA) and BC; (ii) inhibited the SM + NNK-induced phosphorylation of Jun N-terminal kinase (JNK), extracellular-signal-regulated protein kinase (ERK) and proliferating cellular nuclear antigen proteins in the lungs of ferrets; and (iii) blocked the SM + NNK-induced up-regulation of total p53 and Bax proteins, as well as phosphorylated p53 in the lungs of ferrets. In addition, there were no lesions observed in the lung tissue of ferrets in the control and/or the AOX groups after 6 months of intervention, but combined AOX supplementation resulted in a trend toward lower incidence of both preneoplastic lung lesions and lung tumor formation in SM + NNK + AOX group of ferrets, as compared with the SM + NNK group alone. These data indicate that combined AOX supplementation could be a useful chemopreventive strategy against lung carcinogenesis through maintaining normal tissue levels of RA and inhibiting the activation of mitogen-activated protein kinase pathways, cell proliferation and phosphorylation of p53.

Dose-dependence of promotion of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline-induced rat hepatocarcinogenesis by ethanol: evidence for a threshold

Although ethanol is thought to be a tumor-promoter, there are conflicting results concerning its effects on experimental hepatocarcinogenesis. Furthermore, the relationship between the amount of ethanol consumed and tumor promoting effects has hitherto not been investigated in detail. In the present study, 21-day-old F344/DuCrj rats were fed 200 p.p.m. 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) in their diet for 8 weeks and thereafter received ethanol at doses of 0, 0.1, 0.3, 1, 3, 10 and 20% in drinking water ad libitum for 16 weeks. The incidences of hepatocellular adenoma and total tumors increased dose-dependently with statistical significance at doses of 10% and 20%, compared to the initiated control value. Similarly, dose dependence was observed for the incidence of hepatocellular carcinoma, which was elevated significantly at the dose of 20%. No alteration in development of preneoplastic glutathione-S-transferase placental form positive foci or tumors was observed with 0.1-1%. Cell proliferation also increased dose-dependently and CYP2E1 protein induction was recognized in centrilobular regions without alteration in mRNA levels, but no effects were evident on formation of 8-hydroxy-2'-deoxyguanosine, an oxidative DNA damage marker, or lipid peroxidation in any of the initiated groups. The mRNA expression of cyclin D1 increased dose dependently. The results demonstrated that ethanol dose-dependently promotes hepatocarcinogenesis induced by MeIQx, but with no adverse influence at doses of 1% or less, comparable to sensible drinking levels in humans.

Dosage effects of resveratrol on ethanol-induced cell death in the human K562 cell line

Previous studies have established that ethanol induces cell apoptosis and necrosis. However, the precise molecular mechanisms are currently unclear. Here, we show that higher concentrations of ethanol (250-400 mM) induced a shift from apoptotic to necrotic cell death in human K562 cells, and that resveratrol, a grape-derived phytoalexin with known antioxidant and anti-inflammatory properties, inhibited or enhanced ethanol-induced apoptosis/necrosis depending on the treatment dosage. Using the cell permeable dye 2',7'-dichlorofluorescin diacetate (DCF-DA) as an indicator of reactive oxygen species (ROS) generation, we showed that ethanol treatment directly increased intracellular oxidative stress. This intracellular oxidative stress increased in response to high concentrations (100-200 microM) of resveratrol, but remained unchanged following treatment with low concentrations (10-25 microM) of resveratrol. Further studies showed that resveratrol could attenuate or enhance ethanol-induced intracellular oxidative stress generation-dependent on treatment dosage, and that this effect could be correlated with cell apoptosis or necrosis. Importantly, ethanol-induced changes in intracellular ATP levels were also correlated with resveratrol dosage. Taken together, these results indicate that the treatment dosage may determine the effect of resveratrol on ethanol-induced ROS generation, intracellular ATP levels, and cell apoptosis or necrosis. Thus our findings support the possibility that appropriate dosage of resveratrol aids in decreasing the toxic effect of ethanol.

Alcohol, vitamin A, and cancer

Chronic and excessive alcohol intake is associated with an increased risk of a variety of cancers (e.g., oral cavity, larynx, esophagus, liver, lung, colorectal, and breast). Retinoids (vitamin A and its derivatives) are known to exert profound effects on cellular growth, cellular differentiation, and apoptosis, thereby controlling carcinogenesis. Lower hepatic vitamin A levels have been well documented in alcoholics. Substantial research has been done, investigating the mechanisms by which excessive alcohol interferes with retinoid metabolism. More specifically, (1) alcohol acts as a competitive inhibitor of vitamin A oxidation to retinoic acid involving alcohol dehydrogenases and acetaldehyde dehydrogenases; (2) alcohol-induced cytochrome P450 enzymes (CYP), particularly CYP2E1, enhance catabolism of vitamin A and retinoic acid; and (3) alcohol alters retinoid homeostasis by increasing vitamin A mobilization from liver to extrahepatic tissues. As a consequence, long-term and excessive alcohol intake results in impaired status of retinoic acid, the most active derivative of vitamin A and a ligand for both retinoic acid receptors and retinoid X receptors. Moreover, this alcohol-impaired retinoic acid homeostasis interferes with (1) retinoic acid signaling (e.g., down-regulates retinoid target gene expression) and (2) retinoic acid "cross-talk" with the mitogen-activated protein kinase [(MAPK), including Jun N-terminal kinase, extracellular signal-regulated kinase, and p38 kinase] signaling pathway. In addition, restoration of retinoic acid homeostasis by retinoic acid supplementation restored the normal status of both retinoid and MAPK signaling, thereby maintaining normal cell proliferation and apoptosis in alcohol-fed animals. These observations would have implications for the prevention of alcohol-promoted liver (and peripheral tissue) carcinogenesis. However, a better understanding of the alcohol-retinoid interaction and the molecular mechanisms involved is needed before retinoids can be pursued in the prevention of alcohol-related carcinogenesis in human beings, particularly regarding the detrimental effects of polar metabolites of vitamin A.

Hepatotoxicity of alcohol-induced polar retinol metabolites involves apoptosis via loss of mitochondrial membrane potential

Chronic alcohol consumption depletes hepatic vitamin A stores. However, vitamin A supplementation is hepatotoxic, which is further potentiated by concomitant alcohol consumption. It was suggested that polar retinol metabolites generated by alcohol-inducible cytochrome P4502E1 aggravate liver damage. However, experimental evidence supporting this hypothesis is lacking. To elucidate the effects of polar retinol metabolites on cultured HepG2 cells and primary rat hepatocytes, polar retinol metabolites were extracted from liver tissues of rats fed either an alcoholic or isocaloric control Lieber-DeCarli diet. Cell toxicity assays included morphology assessment, trypan blue exclusion test, and LDH/AST leakage. Staining for DAPI and acridine orange, FACS analysis, and Western blot for cleaved caspase-9 and -3 were used to detect apoptosis. Polar retinol metabolites caused marked cytotoxicity in a concentration- and time-dependent manner in both cell types reflected by morphological changes, a dramatic increase in trypan blue positive cells, and LDH/AST leakage. Toxicity was due to apoptosis, as demonstrated by a time-dependent increase of sub-G1 cellular events, a rapid loss of mitochondrial membrane potential, and a time-dependent activation of caspase-9 and -3. No toxicity was found with equivalent doses of the control extract from nonalcoholic rats. We demonstrate that polar retinol metabolites cause marked hepatocyte death through the induction of apoptosis.

Pro- and anti-apoptotic roles of c-Jun N-terminal kinase (JNK) in ethanol and acetaldehyde exposed rat hepatocytes

We have examined the significance of the activation of c-Jun N-terminal kinase (JNK) and p42/44 mitogen-activated protein kinase (MAPK) by ethanol and acetaldehyde in rat hepatocyte apoptosis. Acetaldehyde induced rapid and transient (15 min) activation of p42/44 MAPK followed by activation of JNK, which remained above control up to 1 h. Ethanol activated JNK for up to 4 h. Both ethanol and acetaldehyde caused apoptosis as determined by DNA fragmentation, caspase-3 activation and 2'[4-ethoxyphenyl]-5-[4-methyl-piperazinyl]-2,5'-bi-1H-benzimidazole (Hoechst 33342) staining. Ethanol-induced apoptosis was blocked by JNK inhibitor 1,9-pyrazoloanthrone (SP600125), indicating that JNK activation is pro-apoptotic. In contrast, acetaldehyde-induced apoptosis was not suppressed by this inhibitor. In fact, SP600125 potentiated acetaldehyde-induced apoptosis, suggesting that JNK activation is anti-apoptotic. Inhibition of p42/44 MAPK by MAPK kinase (MKK1) inhibitor, 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene (U0126), potentiated apoptosis by acetaldehyde or ethanol, suggesting anti-apoptotic role of p42/44 MAPK. The activation of JNK by ethanol or acetaldehyde was insensitive to the genistein (tyrosine kinase inhibitor), GF109203X (2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)maleimide, protein kinase C [PKC] inhibitor) and N-acetylcysteine (N-AC) (antioxidant), whereas p42/44 MAPK activation by acetaldehyde was inhibited by genistein and GF109203X. Furthermore, p42/44 MAPK activation is not necessary for the JNK activation. In summary, transient activation of JNK by acetaldehyde is anti-apoptotic, whereas sustained activation of JNK by ethanol is pro-apoptotic. The activation of p42/44 MAPK appears to be anti-apoptotic for both ethanol and acetaldehyde. Thus, JNK activation by ethanol and acetaldehyde can be both pro- and anti-apoptotic in hepatocytes.

Retinoic acid induced repression of AP-1 activity is mediated by protein phosphatase 2A in ovarian carcinoma cells

In previous studies we have shown that all-trans retinoic acid (atRA)-treatment of the atRA-sensitive ovarian carcinoma cell line CA-OV3 repressed AP-1 activity by about 50%, while a similar effect was not observed in the atRA-resistant ovarian carcinoma cell line, SK-OV3. These results suggested that the repression of AP-1 activity may be one of the mechanisms by which atRA inhibits the growth of atRA-sensitive CA-OV3 cells. In the present studies, we investigated further the molecular mechanism by which AP-1 activity is repressed by atRA. We show that the repression of AP-1 activity correlates with an increase in JunB protein expression and a decrease in N-terminal phosphorylation of c-Jun. The decrease in N-terminal phosphorylation of c-Jun does not appear to be modulated by JNK or ERK, since their protein expression patterns and kinase activity do not correlate with the repression of AP-1 activity following treatment with atRA. However, the activity of the protein phosphatase PP2A was found to increase 24 h following atRA treatment in CA-OV3 cells. Moreover, the catalytic subunit of PP2A was found to associate with c-Jun in vivo following atRA treatment. Since the inhibition of AP-1 activity following atRA treatment of CA-OV3 cells was abolished in the presence of specific PP2A inhibitors, it is likely that PP2A plays an important role in the atRA-induced repression of AP-1.

Alcohol-reduced plasma IGF-I levels and hepatic IGF-I expression can be partially restored by retinoic acid supplementation in rats

Chronic and excessive ethanol intake in rats results in low levels of hepatic retinoic acid (RA) either by inhibiting the biosynthesis of RA or by enhancing its catabolism of RA. Chronic ethanol intake also decreases both hepatic expression of insulin-like growth factor-I (IGF-I) and plasma IGF-I concentration in rats. It is not known whether RA supplementation in alcohol-fed rats can restore plasma IGF-I concentrations and hepatic IGF-I expression. In the present study, we examined both plasma IGF-I level and hepatic IGF-I mRNA expression in alcohol-fed rats with or without RA (100 microg/kg body weight) supplementation for 6 mo. Hepatic IGF-I mRNA levels and plasma IGF-I concentration were decreased (84 and 29%, respectively) significantly in alcohol-fed rats compared with the control. In contrast, RA supplementation in ethanol-fed rats partially restored both hepatic IGF-I mRNA levels and plasma IGF-I concentration compared with rats fed ethanol alone. These data suggest that alcohol-impaired hepatic RA status contributes to the decreased plasma IGF-I level and hepatic IGF-I expression in alcoholics.

Low dose beta-carotene supplementation of ferrets attenuates smoke-induced lung phosphorylation of JNK, p38 MAPK, and p53 proteins

We demonstrated previously that smoke exposure and/or high-dose beta-carotene supplementation decreases levels of retinoic acid and retinoic acid receptor beta (RARbeta) protein, but increase levels of c-Jun and proliferating cellular nuclear antigen protein in the lungs of ferrets. In contrast, low-dose beta-carotene can prevent the decreased lung retinoic acid and the smoke-induced lung lesions. In the present study, we investigated whether smoke exposure and/or beta-carotene supplementation could affect Jun N-terminal kinase (JNK), p38 mitogen-activated protein kinase (MAPK), and p53 in the lungs of ferrets. Ferrets were subjected to cigarette smoke exposure and either a high or low dose of beta-carotene (2 x 3 factorial design) for 6 mo. There were greater protein levels of phosphorylated JNK, p38, and c-Jun, but lower levels of MAPK phophatase-1 (MKP-1) in groups exposed to smoke and/or high dose beta-carotene. Both phosphorylated-p53 and total p53 were substantially increased in the lungs of these groups. In contrast, low-dose beta-carotene greatly attenuated the smoke-induced phosphorylation of JNK, p38, c-Jun, p53, and total p53, accompanied by upregulated MKP-1. Smoke exposure increased MAPK kinase-4 (MKK4) phosphorylation regardless of beta-carotene supplementation. These data indicate that restoration of retinoic acid and MKP-1 by low-dose beta-carotene in the lungs of ferrets may prevent the smoke-induced activation of the JNK-dependent signaling pathway, p38 MAPK, and the associated phosphorylation of p53, thereby lowering the risk of the smoke-related lung lesions. These data provide supportive evidence that the beneficial vs. detrimental effects of beta-carotene supplementation are related to the dosage of beta-carotene administered.

Alcohol and cancer: genetic and nutritional aspects

Chronic alcohol consumption is a major risk factor for cancer of upper aero-digestive tract (oro-pharynx, hypopharynx, larynx and oesophagus), the liver, the colo-rectum and the breast. Evidence has accumulated that acetaldehyde is predominantly responsible for alcohol-associated carcinogenesis. Acetaldehyde is carcinogenic and mutagenic, binds to DNA and protein, destroys the folate molecule and results in secondary cellular hyper-regeneration. Acetaldehyde is produced by mucosal and cellular alcohol dehydrogenase, cytochrome P450 2E1 and through bacterial oxidation. Its generation and/or its metabolism is modulated as a result of polymorphisms or mutations of the genes responsible for these enzymes. Acetaldehyde can also be produced by oral bacteria. Smoking, which changes the oral bacterial flora, also increases salivary acetaldehyde. Cigarette smoke and some alcoholic beverages, such as Calvados, contain acetaldehyde. In addition, chronic alcohol consumption induces cytochrome P450 2E1 enxyme activity in mucosal cells, resulting in an increased generation of reactive oxygen species and in an increased activation of various dietary and environmental carcinogens. Deficiencies of riboflavin, Zn, folate and possibly retinoic acid may further enhance alcohol-associated carcinogenesis. Finally, methyl deficiency as a result of multiple alcohol-induced changes leads to DNA hypomethylation. A depletion of lipotropes, including methionine, choline, betaine and S-adenosylmethionine, as well as folate, results in the hypomethylation of oncogenes and may lead to DNA strand breaks, all of which are associated with increased carcinogenesis.

MAP kinase signaling in diverse effects of ethanol

Chronic ethanol abuse is associated with liver injury, neurotoxicity, hypertension, cardiomyopathy, modulation of immune responses and increased risk for cancer, whereas moderate alcohol consumption exerts protective effect on coronary heart disease. However, the signal transduction mechanisms underlying these processes are not well understood. Emerging evidences highlight a central role for mitogen activated protein kinase (MAPK) family in several of these effects of ethanol. MAPK signaling cascade plays an essential role in the initiation of cellular processes such as proliferation, differentiation, development, apoptosis, stress and inflammatory responses. Modulation of MAPK signaling pathway by ethanol is distinctive, depending on the cell type; acute or chronic; normal or transformed cell phenotype and on the type of agonist stimulating the MAPK. Acute exposure to ethanol results in modest activation of p42/44 MAPK in hepatocytes, astrocytes, and vascular smooth muscle cells. Acute ethanol exposure also results in potentiation or prolonged activation of p42/44MAPK in an agonist selective manner. Acute ethanol treatment also inhibits serum stimulated p42/44 MAPK activation and DNA synthesis in vascular smooth muscle cells. Chronic ethanol treatment causes decreased activation of p42/44 MAPK and inhibition of growth factor stimulated p42/44 MAPK activation and these effects of ethanol are correlated to suppression of DNA synthesis, impaired synaptic plasticity and neurotoxicity. In contrast, chronic ethanol treatment causes potentiation of endotoxin stimulated p42/44 MAPK and p38 MAPK signaling in Kupffer cells leading to increased synthesis of tumor necrosis factor. Acute exposure to ethanol activates pro-apoptotic JNK pathway and anti-apoptotic p42/44 MAPK pathway. Apoptosis caused by chronic ethanol treatment may be due to ethanol potentiation of TNF induced activation of p38 MAPK. Ethanol induced activation of MAPK signaling is also involved in collagen expression in stellate cells. Ethanol did not potentiate serum stimulated or Gi-protein dependent activation of p42/44 MAPK in normal hepatocytes but did so in embryonic liver cells and transformed hepatocytes leading to enhanced DNA synthesis. Ethanol has a 'triangular effect' on MAPK that involve direct effects of ethanol, its metabolically derived mediators and oxidative stress. Acetaldehyde, phosphatidylethanol, fatty acid ethyl ester and oxidative stress, mediate some of the effects seen after ethanol alone whereas ethanol modulation of agonist stimulated MAPK signaling appears to be mediated by phosphatidylethanol. Nuclear MAPKs are also affected by ethanol. Ethanol modulation of nuclear p42/44 MAPK occurs by both nuclear translocation of p42/44 MAPK and its activation in the nucleus. Of interest is the observation that ethanol caused selective acetylation of Lys 9 of histone 3 in the hepatocyte nucleus. It is plausible that ethanol modulation of cross talk between phosphorylation and acetylations of histone may regulate chromatin remodeling. Taken together, these recent developments place MAPK in a pivotal position in relation to cellular actions of ethanol. Furthermore, they offer promising insights into the specificity of ethanol effects and pharmacological modulation of MAPK signaling. Such molecular signaling approaches have the potential to provide mechanism-based therapy for the management of deleterious effects of ethanol or for exploiting its beneficial effects.

Hepatitis C virus core protein activates ERK and p38 MAPK in cooperation with ethanol in transgenic mice

In human chronic hepatitis C, alcohol intake is a synergistic factor for the acceleration of hepatocarcinogenesis. Recently, we showed a significant increase of reactive oxygen species (ROS) in hepatitis C virus (HCV) core-transgenic mice fed ethanol-containing diets. Because previous studies indicated that ROS is closely associated with mitogen-activated protein kinases (MAPK), we examined activities of c-Jun N-terminal kinase, p38 MAPK, and extracellular signal-regulated kinase (ERK) in the liver of core-transgenic and nontransgenic mice with short-term ethanol feeding. Activity of ERK and p38 MAPK was increased in core-transgenic mice compared with nontransgenic mice, whereas neither ERK nor p38 MAPK was activated in core-transgenic mice with normal diets. In addition, activity of cyclic-AMP and serum responsive element, downstream pathways of p38 MAPK and ERK, was also increased. Comparison of gene expression profiles by cDNA microarray and real-time PCR revealed that galectin-1, which is associated with cell transformation, was significantly increased in ethanol-fed core-transgenic mice. On the other hand, glutathione S-transferase (GST), which plays a key role in protecting cells from oxidative stress, was decreased. In conclusion, these results suggest that HCV core protein cooperates with ethanol for the activation of some MAPK pathways, and leads to the modulation of several genes, contributing to the pathogenesis of liver disease of HCV-infected patients with high ethanol consumption.

The future of GI and liver research: editorial perspectives. III. JNK/AP-1 regulation of hepatocyte death

Activation of the JNK/activator protein-1 (AP-1)-signaling pathway is a common mediator of hepatocyte death from a variety of stimuli. Although the mechanism by which JNK or AP-1 promotes death is unknown, it results when activation of this signaling pathway is unusually prolonged. Although JNK/AP-1 mediates TNF-induced cell death at or above the level of the mitochondria, the ability of JNK/AP-1 to promote death from necrosis as well as apoptosis suggests that JNK/AP-1 may induce death by several mechanisms. Recognition of JNK/AP-1 signaling as a critical promoter of hepatocyte death raises the possibility that the therapeutic manipulation of this pathway may be effective in the treatment of human liver disease.

Enhancing risk of ethanol on MeIQx-induced rat hepatocarcinogenesis is accompanied with increased levels of cellular proliferation and oxidative stress

We investigated promotion potential of ethanol after initiation of hepatocarcinogenesis in male, 21-day-old, F344 rats by exposure to 10 ppm 2-amino-3, 8-dimethylimidazo[4,5-f]quinoxaline pellet diet for 8 weeks. The rats in group 1 were then fed on liquid control diet for 16 weeks, group 2 receiving the same diet containing 5% ethanol for 8 weeks followed by 8 weeks on the control diet, while group 3 animals were given 5% ethanol containing liquid diet for the entire16 weeks. On sacrifice at the end of week 24, glutathione S-transferase placental form positive foci, putative preneoplastic lesions in the liver, cell proliferation as indicated by proliferating cell nuclear antigen immunohistochemical staining and levels of 8-hydroxydeoxyguanosine, a marker of oxidative DNA damage, were significantly increased in the liver of group 3 along with non significant alteration of 8-oxoguanine DNA glycosylase mRNA expression. Lack of persistent increase of above parameters was found in transient ethanol exposure group. These results suggest that chronic consumption of ethanol promotes hepatocarcinogenesis by increasing oxidative stress and cell proliferation. It is also evident that abstinence of ethanol during the second stage stops its persistent promotion effect.

Retinoids and alcohol-related carcinogenesis

Chronic and excessive alcohol intake is associated with an increased incidence of a variety of cancers (e.g., liver, oral cavity, esophagus, colorectal and breast). Long-term alcohol intake results in impaired nutritional status of retinoic acid (RA), the most active derivative of vitamin A, which may provide a promoting environment for tumor formation. Recent studies demonstrate that chronic alcohol-induced hepatocellular proliferation, which may convert hepatocytes from a state of resistance to a carcinogen to a state of high susceptibility, is due to alcohol-impaired RA metabolism and signaling and crosstalk with the Jun N-terminal kinases-dependent signaling pathway. Further, the restoration of hepatic RA homeostasis by treatment with either RA supplementation or inhibitors of RA catabolism can suppress alcohol-induced hepatocyte hyperproliferation and restore alcohol-deregulated apoptosis, thereby reducing the risk of alcohol-promoted hepatocellular carcinogenesis. These studies indicate the importance of RA actions in the prevention and/or treatment of alcohol-related carcinogenic process in the liver and other organs.