Erwin Gäbele

NADPH oxidase-derived free radicals are key oxidants in alcohol-induced liver disease

In North America, liver disease due to alcohol consumption is an important cause of death in adults, although its pathogenesis remains obscure. Despite the fact that resident hepatic macrophages are known to contribute to early alcohol-induced liver injury via oxidative stress, the exact source of free radicals has remained a mystery. To test the hypothesis that NADPH oxidase is the major source of oxidants due to ethanol, we used p47(phox) knockout mice, which lack a critical subunit of this major source of reactive oxygen species in activated phagocytes. Mice were treated with ethanol chronically, using a Tsukamoto-French protocol, for 4 weeks. In wild-type mice, ethanol caused severe liver injury via a mechanism involving gut-derived endotoxin, CD14 receptor, production of electron spin resonance-detectable free radicals, activation of the transcription factor NF-kappaB, and release of cytotoxic TNF-alpha from activated Kupffer cells. In NADPH oxidase-deficient mice, neither an increase in free radical production, activation of NF-kappaB, an increase in TNF-alpha mRNA, nor liver pathology was observed. These data strongly support the hypothesis that free radicals from NADPH oxidase in hepatic Kupffer cells play a predominant role in the pathogenesis of early alcohol-induced hepatitis by activating NF-kappaB, which activates production of cytotoxic TNF-alpha.

The role of Kupffer cell oxidant production in early ethanol-induced liver disease.

Considerable evidence for a role of Kupffer cells in alcoholic liver disease has accumulated and they have recently been shown to be a predominant source of free radicals. Several approaches including pharmacological agents, knockout mice, and viral gene transfer have been used to fill critical gaps in understanding key mechanisms by which Kupffer cell activation, oxidant formation, and cytokine production lead to liver damage and subsequent pathogenesis. This review highlights new data in support of the hypothesis that Kupffer cells play a pivotal role in hepatotoxicity due to ethanol by producing oxidants via NADPH oxidase.

Role of Lipopolysaccharide-Binding Protein in Early Alcohol-Induced Liver Injury in Mice

Cellular responses to endotoxins are enhanced markedly by LPS-binding protein (LBP). Furthermore, it has been demonstrated that endotoxins and proinflammatory cytokines such as TNF-α participate in early alcohol-induced liver injury. Therefore, in this study, a long-term intragastric ethanol feeding model was used to test the hypothesis that LBP is involved in alcoholic hepatitis by comparing LBP knockout and wild-type mice. Two-month-old female mice were fed a high-fat liquid diet with either ethanol or isocaloric maltose-dextrin as control continuously for 4 wk. There was no difference in mean urine alcohol concentrations between the groups fed ethanol. Dietary alcohol significantly increased liver to body weight ratios and serum alanine aminotransferase levels in wild-type mice (189 ± 31 U/L) over high-fat controls (24 ± 7 U/L), effects which were blunted significantly in LBP knockout mice (60 ± 17 U/L). Although no significant pathological changes were observed in high-fat controls, 4 wk of dietary ethanol caused steatosis, mild inflammation, and focal necrosis in wild-type animals as expected (pathology score, 5.9 ± 0.5). These pathological changes were reduced significantly in LBP knockout mice fed ethanol (score, 2.6 ± 0.5). Endotoxin levels in the portal vein were increased significantly after 4 wk in both groups fed ethanol. Moreover, ethanol increased TNF-α mRNA expression in wild-type, but not in LBP knockout mice. These data are consistent with the hypothesis that LBP plays an important role in early alcohol-induced liver injury by enhancing LPS-induced signal transduction, most likely in Kupffer cells.

Systemic infusion of angiotensin II exacerbates liver fibrosis in bile duct–ligated rats

Recent evidence indicates that the renin–angiotensin system (RAS) plays a major role in liver fibrosis. Here, we investigate whether the circulatory RAS, which is frequently activated in patients with chronic liver disease, contributes to fibrosis progression. To test this hypothesis, we increased circulatory angiotensin II (Ang II) levels in rats undergoing biliary fibrosis. Saline or Ang II (25 ng/kg/h) were infused into bile duct–ligated rats for 2 weeks through a subcutaneous pump. Ang II infusion increased serum levels of Ang II and augmented bile duct ligation–induced liver injury, as assessed by elevated liver serum enzymes. Moreover, it increased the hepatic concentration of inflammatory proteins (tumor necrosis factor α and interleukin 1β) and the infiltration of CD43‐positive inflammatory cells. Ang II infusion also favored the development of vascular thrombosis and increased the procoagulant activity of tissue factor in the liver. Livers from bile duct–ligated rats infused with Ang II showed increased transforming growth factor β1 content, collagen deposition, accumulation of smooth muscle α‐actin–positive cells, and lipid peroxidation products. Moreover, Ang II infusion stimulated phosphorylation of c‐Jun and p42/44 mitogen‐activated protein kinase and increased proliferation of bile duct cells. In cultured rat hepatic stellate cells (HSCs), Ang II (10−8 mol/L) increased intracellular calcium and stimulated reactive oxygen species formation, cellular proliferation and secretion of proinflammatory cytokines. Moreover, Ang II stimulated the procoagulant activity of HSCs, a newly described biological function for these cells. In conclusion, increased systemic Ang II augments hepatic fibrosis and promotes inflammation, oxidative stress, and thrombogenic events. (HEPATOLOGY 2005;41:1046–1055.)

Liver fibrosis: signals leading to the amplification of the fibrogenic hepatic stellate cell.

Liver fibrosis represents a major medical problem with significant morbidity and mortality. Worldwide hepatitis viral infections represent the major cause liver fibrosis; however, within the United States chronic ethanol consumption is the leading cause of hepatic fibrosis. Other known stimuli for liver fibrosis include helminthic infection, iron or copper overload and biliary obstruction. Fibrosis can be classified as a wound healing response to a variety of chronic stimuli that is characterized by an excessive deposition of extracellular matrix proteins of which type I collagen predominates. This excess deposition of extracellular matrix proteins disrupts the normal architecture of the liver resulting in pathophysiological damage to the organ. If left untreated fibrosis can progress to liver cirrhosis ultimately leading to organ failure and death if left untreated. This review will discuss the molecular events leading to liver fibrosis. The discussion will include collagen gene regulation and proliferative signals that contribute to the amplification of the hepatic stellate cell, the primary fibrogenic cell type that resides in the liver.

Cocoa extract protects against early alcohol-induced liver injury in the rat

Oxidants have been shown to be involved in alcohol-induced liver injury. This study was designed to determine whether cocoa flavonoid extract, composed mostly of epicatechin and epicatechin oligomers, protects against early alcohol-induced liver injury in rats. Male Wistar rats were fed high-fat liquid diets with or without ethanol (10-14 g/kg per day) and cocoa extract (400 mg/kg per day) continuously for 4 weeks using an enteral feeding protocol. Mean body weight gains ( approximately 4 g/day) were not significantly different between treatment groups. Cocoa extract did not affect average daily urine ethanol concentrations ( approximately 200mg/dL). After 4 weeks, serum alanine amino transferase levels of the ethanol group were increased nearly fourfold (110+/-16 IU/L) compared to control values (35+/-3 IU/L); this effect of ethanol was blocked by cocoa extract (60+/-6 IU/L). Additionally, enteral ethanol caused severe fat accumulation, mild inflammation, and necrosis in the liver; cocoa extract significantly blunted these changes. Increases in liver TNFalpha protein levels caused by ethanol were completely blocked by cocoa extract. Further, ethanol significantly increased the accumulation of protein adducts of 4-hydroxynonenal, a product of lipid peroxidation serving as an index of oxidative stress; again this was counteracted by the addition of cocoa extract. These results indicate that dietary flavanols such as those found in cocoa can prevent early alcohol-induced liver injury.

The role of p70S6K in hepatic stellate cell collagen gene expression and cell proliferation

During fibrosis the hepatic stellate cell (HSC) undergoes a complex activation process characterized by increased proliferation and extracellular matrix deposition. The 70-kDa ribosomal S6 kinase (p70S6K) is activated by mitogens, growth factors, and hormones in a phosphatidylinositol 3-kinase-dependent manner. p70S6K regulates protein synthesis, proliferation, and cell cycle control. Because these processes are involved in HSC activation, we investigated the role of p70S6K in HSC proliferation, cell cycle control, and type I collagen expression. Platelet-derived growth factor (PDGF) stimulated p70S6K phosphorylation, which was blocked by LY294002, an inhibitor of phosphatidylinositol 3-kinase. Rapamycin blocked phosphorylation of p70S6K but had no affect on PDGF-induced Akt phosphorylation, positioning p70S6K downstream of Akt. Transforming growth factor-β, which inhibits HSC proliferation, did not affect PDGF-induced p70S6K phosphorylation. Rapamycin treatment did not affect α1(I) collagen mRNA but reduced type I collagen protein secretion. Expression of smooth muscle α-actin was not affected by rapamycin treatment, indicating that HSC activation was not altered. Rapamycin inhibited serum-induced DNA synthesis ∼2-fold. Moreover, rapamycin decreased expression of cyclins D1, D3, and E but not cyclin D2, Rb-Ser780, and Rb-Ser795. Together, p70S6K plays a crucial role in HSC proliferation, collagen expression, and cell cycle control, thus representing a potential therapeutic target for liver fibrosis.

TNFα is required for cholestasis-induced liver fibrosis in the mouse

TNFalpha, a mediator of hepatotoxicity in several animal models, is elevated in acute and chronic liver diseases. Therefore, we investigated whether hepatic injury and fibrosis due to bile duct ligation (BDL) would be reduced in TNFalpha knockout mice (TNFalpha-/-). Survival after BDL was 60% in wild-type mice (TNFalpha+/+) and 90% in TNFalpha-/- mice. Body weight loss and liver to body weight ratios were reduced in TNFalpha-/- mice compared to TNFalpha+/+ mice. Following BDL, serum alanine transaminases (ALT) levels were elevated in TNFalpha+/+ mice (268.6+/-28.2U/L) compared to TNFalpha-/- mice (105.9U/L+/-24.4). TNFalpha-/- mice revealed lower hepatic collagen expression and less liver fibrosis in the histology. Further, alpha-smooth muscle actin, an indicator for activated myofibroblasts, and TGF-beta mRNA, a profibrogenic cytokine, were markedly reduced in TNFalpha-/- mice compared to TNFalpha+/+ mice. Thus, our data indicate that TNFalpha induces hepatotoxicity and promotes fibrogenesis in the BDL model.

Green tea extract protects against early alcohol-induced liver injury in rats

Abstract Oxidants have been shown to be involved in alcoholinduced liver injury. This study was designed to test the hypothesis that the antioxidant polyphenolic extract of green tea, comprised predominantly of epigallocatechin gallate, protects against early alcoholinduced liver injury in rats. Male Wistar rats were fed highfat liquid diets with or without ethanol (10 14 g kg 1 day 1) and green tea (300 mg kg 1 day 1) continuously for 4 weeks using an intragastric enteral feeding protocol. Mean body weight gains (~4 g/day) were not significantly different between treatment groups, and green tea extract did not the affect average concentration or the cycling of urine ethanol concentrations (0 550 mg dl 1 day 1). After 4 weeks, serum ALT levels were increased significantly about 4-fold over control values (35±3 IU/l) by enteral ethanol (114±18); inclusion of green tea extract in the diet significantly blunted this increase (65±10). Enteral ethanol also caused severe fatty accumulation, mild inflammation, and necrosis in the liver. While not affecting fat accumulation or inflammation, green tea extract significantly blunted increases in necrosis caused by ethanol. Furthermore, ethanol significantly increased the accumulation of protein adducts of 4-hydroxynonenal, a product of lipid peroxidation and an index of oxidative stress; green tea extract blocked this effect almost completely. TNFa protein levels were increased in liver by alcohol; this phenomenon was also blunted by green tea extract. These results indicate that simple dietary antioxidants, such as those found in green tea, prevent early alcoholinduced liver injury, most likely by preventing oxidative stress.