Molecular biological aspects of alcohol-induced liver disease

Ethanol consumption alters expression and colocalization of bile salt export pump and multidrug resistance protein 2 in the rat

Chronic ethanol consumption elicits detrimental changes of liver metabolism. By employing a 12-week-long feeding regimen, we investigated the effects of chronic ethanol consumption on the expression and localization of bile salt export pump (Bsep), a major canalicular exporter of bile salts, and multidrug resistance protein 2 (Mrp2), a canalicular organic anion transporter, in the rat liver. RT-PCR, confocal immunofluorescence microscopy, immunoblotting, and quantitative colocalization analysis were used to examine their gene and protein expression, and changes in the distribution of antigenic sites. Bsep mRNA was upregulated, while Mrp2 mRNA responded by downregulation. In agreement with mRNA, the expression of Bsep protein increased, while the expression of Mrp2 protein responded with a decrease, suggesting that the expression of both of them is transcriptionally regulated. Confocal immunofluorescence microscopy showed disruption of the colocalization of Bsep and Mrp2 proteins at the hepatocyte canalicular membrane and their relocation intracellularly. Quantitative colocalization analysis of Bsep and Mrp2 proteins revealed a steady decrease in the degree of colocalization and Mrp2 expression, indicating that although the properties of both transporters are affected, Mrp2 is altered more. These findings provide evidence that ethanol alters Bsep and Mrp2 canalicular transporters in the rat liver, at both the mRNA and protein levels. Mrp2 shows deeper involvement. Eight weeks appears to be a critical time point in this process.

Toxicity of hepatotoxins: new insights into mechanisms and therapy

Liver injury caused by hepatotoxins, such as carbon tetrachloride (CCl4), ethanol, and acetaminophen (APAP), is characterised by varying degrees of hepatocyte degeneration and cell death via either apoptosis or necrosis. The generation of reactive intermediate metabolites from the metabolism of hepatotoxins, and the occurrence of reactive oxygen species (ROS) during the inflammatory reaction account for a variety of pathophysiologic pathways leading to cell death, such as covalent binding, disordered cytosolic calcium homeostasis, glutathione (GSH) depletion, onset of mitochondrial permeability transition (MPT) and associated lipid peroxidation. The metabolism of hepatotoxins by cytochrome P-450 enzyme subtypes is a key step of the intoxication; therefore, enzyme inhibitors are shown to minimise the hepatotoxin-associated liver damage. Understanding the function of transcription factors, such as nuclear factor kappaB (NF-kappaB) in acute liver injury, may provide some answers as to the molecular mechanisms of toxic insults. Moreover, substantial evidence exists that MPT is involved in ROS-associated hepatocellular injury and new findings offer a novel therapeutic approach to attenuate cell damage by blocking the onset of MPT. Thus, oxidant stress and lipid peroxidation are crucial elements leading to hepatotoxin-associated liver injury. In addition to specific treatment for a given hepatotoxin, the general strategy for prevention and treatment of the damage includes reducing the production of reactive metabolites of the hepatotoxins, using anti-oxidative agents, and selectively targeting therapeutics to Kupffer cells or hepatocytes for on-going processes, which play a role in mediating a second phase of the injury.

A pilot study of the safety and tolerability of etanercept in patients with alcoholic hepatitis

BACKGROUND

Alcoholic hepatitis is a cause of major morbidity and mortality, and effective therapeutic regimens to treat this condition are lacking. Both experimental and clinical evidence indicates that tumor necrosis factor alpha (TNF), and the downstream cytokine interleukin-6 (IL-6), correlate with disease severity and may contribute to the pathogenesis and clinical sequelae of alcoholic hepatitis, thereby implicating a possible role for inhibition of TNF in the treatment of alcoholic hepatitis.

OBJECTIVE

The aim of the current study was to assess the safety and tolerability of a p75-soluble TNF receptor:FC fusion protein (etanercept), an agent that binds and neutralizes soluble TNF in patients with alcoholic hepatitis in the form of an open-label pilot trial.

METHODS

Etanercept administration was targeted for 2 wk duration in 13 patients with moderate or severe alcoholic hepatitis as assessed by a discriminant function value greater than 15 and/or the presence of spontaneous hepatic encephalopathy.

CONCLUSIONS

On an intention-to-treat basis, the 30-day survival rate of patients receiving etanercept was 92% (12/13). Adverse events that were encountered included infection, hepatorenal decompensation, and GI bleeding, which required premature discontinuation of etanercept in 23% of patients (3/13). This is the first study to examine TNF inhibition with etanercept in patients with alcoholic hepatitis and the results of this study support the rationale for larger controlled studies to further assess safety and efficacy.

Ethanol inhibits the JAK-STAT signaling pathway in freshly isolated rat hepatocytes but not in cultured hepatocytes or HepG2 cells: evidence for a lack of involvement of ethanol metabolism

OBJECTIVES

To understand the molecular mechanism underlying alcoholic liver injury, effects of acute ethanol on the Janus kinase-signal transducer and activator transcription factor (JAK-STAT) signaling in hepatic cells were studied.

DESIGNS AND METHODS

Effects of acute ethanol on the JAK-STAT signaling in freshly isolated, cultured rat hepatocytes, and HepG2 cells were explored.

RESULTS

Acute ethanol exposure inhibited IL-6- or IFN-activated STAT in freshly isolated hepatocytes but not in cultured hepatocytes, HepG2 cells, or HepG2 cells transfected with alcohol dehydrogenase (ADH) or cytochrome P450(2E1). The inhibitory action of ethanol in freshly isolated hepatocytes was not antagonized by the ADH inhibitor 4-methylpyrazole (4-MP). Acute exposure of hepatocytes to acetaldehyde or hydrogen peroxide did not suppress STAT activation. Further studies indicated that the loss of response to the inhibitory effect of ethanol was not due to hepatocyte proliferation and collagen contact.

CONCLUSIONS

Freshly isolated hepatocytes are more susceptible to the inhibitory action of ethanol on the JAK-STAT signaling than cultured hepatocytes or HepG2 cells, which may be implicated in pathogenesis and progression of alcoholic liver disease.

Study of polymorphisms in the CYP2E1 gene in patients with alcoholic pancreatitis

Cytochrome P450IIEI (CYP2E1) is an ethanol-inducible enzyme. Recently, several novel polymorphisms in the CYP2E1 gene have been identified. A polymorphism at position -35 [G(-35)T] appears to be of functional significance in transcription assays. The aim of this study was to investigate if this and other polymorphisms, at position -1019 [C(-1019)T], 4808 [G(4808)A], and 7668 [T(7668)A] of the CYP2E1 gene are associated with alcoholic pancreatitis. DNA was extracted from peripheral blood of 38 patients with alcoholic chronic pancreatitis (CP), 19 patients with alcoholic acute pancreatitis (AP), 46 alcoholic controls (AC), and 155 normal controls (NC). The polymorphisms were examined by digestion with the corresponding restriction endonucleases following PCR amplification. The results have shown that the frequencies of the rare alleles of these polymorphisms were not significantly different between the CP, AP, and AC groups and NC. Therefore, our study results suggest to us that the polymorphisms investigated in the CYP2E1 gene are unlikely to be involved in the susceptibility and pathogenesis of alcoholic pancreatitis.

Nuclear activation and translocation of mitogen-activated protein kinases modulated by ethanol in embryonic liver cells

Activation and nuclear translocation of mitogen activated protein (MAP) kinases in ethanol-treated embryonic liver cells (BNLCL2) was investigated. The relative amount of MAPK proteins, MAP kinase activity and MAPK/LDH (lactate dehydrogenase) ratios were determined in nuclear and cytosolic fractions before and after serum stimulation. In ethanol-treated cells, serum-stimulated MAPK activation was potentiated in both cytosolic and nuclear fractions. Levels of both the p42 and p44 MAPK proteins increased in nuclear fractions from cells treated with ethanol alone for 24 h. Serum-stimulated nuclear translocation of both p42 and p44 MAPK was potentiated in ethanol-treated cells. Nuclear fractions from ethanol-treated cells had a modest increase in MAP kinase activity concurrent with the increased MAPK protein levels. The ratio of MAPK/LDH increased in nuclear fractions with increasing concentrations of ethanol and after serum stimulation. This further confirmed the nuclear translocation of MAPK and also demonstrated that it is not a non-specific effect of ethanol. These results demonstrate, for the first time, that in BNLCL2 liver cells ethanol treatment has dual effects. First, ethanol triggered nuclear translocation of MAPK without causing its activation. Second, it potentiated serum-stimulated activation and translocation of MAPK in the nucleus. These findings provide a novel mechanism through which ethanol may affect cellular and nuclear processes in liver cells.

Genome polymorphism and alcoholism

Different gene variants have been identified as risk or protective factors in alcoholism. The genes coding for dopamine receptors, serotonin transporters, and dehydrogenases represent susceptibility loci for addictive behaviour. However, alcoholism represents a complex psychiatric symptomatology which is caused by multiple factors, both genetic and environmental. Furthermore, there are probably different subtypes of alcoholism each with a distinct pathophysiology, and thus a different genetic background. Genetic research can help to identify such subtypes, which may require different therapeutic approaches. However, gene polymorphisms are not only responsible for a predisposition to alcoholism, but also for personality traits which influence the likelihood of developing addictive behaviour. Moreover, genetic polymorphisms are probably involved in the way an individual responds to treatment. Also, the severity of secondary diseases resulting from chronic alcohol uptake may depend on the genetic makeup of an individual. New treatment strategies focusing on genes contributing towards drug and alcohol dependence (such as gene therapy) are already under examination in animal models. However, further research is required before these developments will considerably change today's clinical handling of alcoholism.

Nutritional and metabolic effects of alcoholism: their relationship with alcoholic liver disease

Excessive alcohol ingestion disturbs the metabolism of most nutrients. Although alcohol can lead to severe hypoglycemia, alcoholics are usually glucose intolerant, probably due to a inhibition of glucose-stimulated insulin secretion. Ethanol intake also leads to negative nitrogen balance and an increased protein turnover. Alcohol also alters lipid metabolism, causing a profound inhibition of lipolysis. Looking for an association between alcohol intake, nutrition, and alcoholic liver disease, we have observed a higher prevalence of subclinical histologic liver damage among obese alcoholics. Multivariate analysis in a large group of alcoholics has shown that obesity is an independent predictor of alcoholic liver disease. Other authors have reported that alcoholics with a history of obesity have a two to three times higher risk of having alcoholic liver disease than non-obese alcoholics. The possible explanation for this association is that the microsomal system, which plays an important pathogenic role in alcoholic liver disease, is induced in non-alcoholic obese subjects and alcoholics. Also, peripheral blood monocyte cells of obese alcoholics produce higher levels of interleukin-1, a cytokine that can contribute to liver damage. The ingestion of polyunsaturated fatty acids can also increase the damaging effects of alcohol on the liver, as has been demonstrated in rats subjected to continuous intragastric infusion of alcohol. Observations in human alcoholics have shown that liver damage is associated with a higher ratio of C:18:1/C:18:0 and a lower ratio of C:22:4/C:18:2 in liver lipids, consistent with an induction of delta 9 desaturase and an increased peroxidation of C:22:4.

IL-6, IFN-gamma and TNF-alpha production by liver-associated T cells and acute liver injury in rats administered concanavalin A

The relationship between the development of acute hepatitis and the production of TNF-alpha IFN-gamma and IL-6 by liver-associated T lymphocytes following intravenous injection of concanavalin A (Con A) was studied in rats. Following a single injection of Con A, there was a dose and time-dependent correlation in the serum levels of serum alanine aminotransferase (ALT), IL-6, IFN-gamma and TNF-alpha. These increases correlated with an increase in the numbers of CD4+, CD8+ and CD25+ T cells in blood and CD4+ and CD25+ T cells in the liver perfusate, but not with CD8+ T cells in liver perfusate. Increased levels of IL-6, IFN-gamma and TNF-alpha were constitutively produced by liver-associated CD4+ T cells when cultured. In Con A-stimulated cultures, liver-associated CD4+ T cells secreted increasing levels of TNF-alpha in a time-dependent manner following Con A injection, but TNF-alpha production by peripheral blood lymphocytes was transient with peak levels detected at 1 h which then declined over 24 h. Histological examination of the liver revealed fatty change, hepatocyte degeneration and necrosis, with an associated cell infiltrate of neutrophils and CD4+ T cells both in the portal areas and around the central veins. These results support the hypothesis that Con A-induced liver damage is mediated by CD4+ T cells acting within the liver, at least in part through the secretion of TNF-alpha, IFN-gamma and IL-6.

Zonated expression of cytokines in rat liver: effect of chronic ethanol and the cytochrome P450 2E1 inhibitor, chlormethiazole

The release of proinflammatory cytokines by endotoxins and during oxidative stress is considered to be an early key step in the pathogenesis of alcoholic liver disease (ALD). Ethanol-inducible cytochrome P450 2E1 (CYP2E1) has potentially pro-oxidative and toxicological properties, and its expression is restricted to the perivenous region of liver. We investigated zonal differences of cytokine expression in rat liver and how these are affected by alcohol exposure and by chlormethiazole (CMZ), a transcriptional and posttranslational inhibitor of hepatic CYP2E1. Periportal and perivenous cell lysates were obtained by the digitonin pulse technique from livers of rats treated with ethanol and CMZ for 38 days. Cytokine expression on the mRNA and protein levels was quantified using competitive polymerase chain reaction (PCR) and Western blot, respectively. Chronic ethanol treatment significantly increased the expression of CYP2E1, microsomal p-nitrophenol hydroxylase activity (indicative for CYP2E1 enzyme activity), and the expression of transforming growth factor beta1 (TGF-beta1), tumor necrosis factor alpha (TNF-alpha), and interleukin (IL)-1beta (1.4- to 4.6-fold). In contrast, ethanol caused a decrease in IL-4 expression and had no influence on IL-6 expression. CMZ treatment caused a reduction in hepatic CYP2E1 expression and in the ethanol-induced cytokine expression by 40% to 60%. Expression of IL-6, IL-2, and IL-4 mRNA occurred preferentially in the periportal region, whereas ethanol caused a pronounced increase in the perivenous expression of TGF-beta1, which was inhibited by CMZ as monitored both on the mRNA and protein levels. These results show the zonated expression of several cytokines and the counteraction of CMZ on all effects of ethanol on cytokine expression. The data further strengthen a link between increased CYP2E1 expression and enhanced cytokine expression as important events in the development of ALD.

Polymorphism of alcohol-metabolizing genes affects drinking behavior and alcoholic liver disease in Japanese men

Alcohol is known to be mainly metabolized in the liver by alcohol dehydrogenase 2 (ADH2) and aldehyde dehydrogenase 2 (ALDH2), and cytochrome P-450IIEI. The purpose of this study was to clarify the role of polymorphism of these ethanol-metabolizing enzymes in drinking behavior and the progression of alcoholic liver disease among Japanese men. Polymorphism of the ADH2, ALDH2, and P-45IIEI genes were determined by polymerase chain reaction, followed by restriction fragment-length polymorphism analysis in 189 normal Japanese men and 26 male patients with alcoholic liver disease. Drinking behavior was estimated by self-assessment according to DSM-III-R criteria. Facial flushing was reported in 91 subjects heterozygous for ALDH2*1/*2 and in two subjects homozygous for ALDH2*2/*2, but was not found in 96 subjects homozygous for ALDH2*1/*1. In contrast, polymorphism of ADH2 and P-450IIEI did not differ between flushers and nonflushers. Although the flushers only drank a small amount of alcohol (< 20 g of ethanol/day), the nonflushers were divided into a group of moderate drinkers (20 to 80 g/day; n = 54) and a group of heavy drinkers (> 80 g/day; n = 42). A high preponderance of heterozygosity for the ADH2*1/*2 genes (20/42; 60%) and a high frequency of the ADH2*1 allele were found in heavy drinkers, compared with moderate drinkers. However, cytochrome P-45IIEI gene polymorphism was similar among the moderate and heavy drinkers. Not only a high frequency of the ALDH2*1 and ADH2*1 alleles, but also a high frequency of the P-450IIEI c2 allele was found in the patients with alcoholic liver disease. From these results, the drinking behavior of Japanese men is strongly influenced by the ALDH2*1 allele, and the level of alcohol intake is affected by the ADH2*1 allele, but not by cytochrome P-45IIEI. However, progression to alcoholic liver disease among heavy drinkers may be affected by the cytochrome P-450IIEI c2 allele.

Effects of chronic ethanol administration on the endocytosis of cytokines by rat hepatocytes

The effects of chronic ethanol administration on the endocytosis of three representative cytokines were investigated in isolated rat hepatocytes. When hepatocytes were isolated from rats that were fed an ethanol liquid diet for 12 to 13 weeks, these cells exhibited a decreased ability to internalize and degrade transforming growth factor-alpha, tumor necrosis factor-alpha and interleukin-6, compared with hepatocytes from the pair-fed controls. This impaired endocytosis of all three cytokines was accompanied by significant decreases in the amount of hepatocyte surface-bound cytokine. Changes in cytokine binding to surface receptors and reduced rates of receptor-cytokine complex internalization into the cells seem to be major contributors to defective endocytosis in hepatocytes from the ethanol-fed rats. Impaired hepatocyte endocytosis could lead to altered steady-state levels of cytokines in the liver and modified physiological responses to cytokines. These changes could affect homeostasis among the various cell types in the liver and could contribute to liver dysfunction and injury.