Luping Guo

Metformin Prevents Endotoxin-Induced Liver Injury after Partial Hepatectomy

Metformin [2-(N,N-dimethylcarbamimidoyl)guanidine] is a drug used in the treatment of type 2 diabetes. Recent studies have suggested that metformin may have effects in addition to lowering serum glucose concentrations (e.g., anti-inflammatory). The aim of the present study was to determine whether metformin prevents the inflammatory reaction and liver damage in a model of postsurgical sepsis. Accordingly, rats underwent 2/3 partial hepatectomy (PH; or sham surgery); 48 h after surgery, animals were administered endotoxin (LPS; 1.5 mg/kg i.v.). Both PH and LPS alone caused some minor liver damage. However, their combined effect (PH/LPS) was synergistic, leading to robust hepatic damage, as indicated by plasma enzymes and histological assessment. Although metformin treatment did not alter changes caused by PH alone, it almost completely blunted the effects of LPS in the PH/LPS group. Increases in biomarkers of inflammation (e.g., interleukin 6, interferon γ, and neutrophil number) were also blunted by metformin treatment. Furthermore, PH/LPS caused a >200× increase in hepatic plasminogen activator inhibitor 1 (PAI-1) mRNA expression and plasma PAI-1 protein. These increases were associated with inhibition of hepatic urokinase plasminogen activator activity and an increase in fibrin deposition, indicative of local thrombosis. These effects were markedly reduced by metformin treatment. In conclusion, these data demonstrate that metformin prevents liver damage in a model of postsurgical sepsis in rats by decreasing proinflammatory and hemostatic responses.

Role of the renin‐angiotensin system in hepatic ischemia reperfusion injury in rats

It has been shown that the renin‐angiotensin system (RAS) plays key roles in the development of fibrosis in numerous organs, including the liver. Other studies have suggested that the RAS also may play roles in diseases of chronic inflammation. However, whether the RAS also can mediate acute inflammation in liver is unclear. The purpose of this study therefore was to determine the effect of the RAS inhibitors captopril and losartan on acute liver damage and inflammation caused by hepatic ischemia and subsequent reperfusion. Accordingly, male rats were subjected to 1 hour of hepatic ischemia (70%) followed by reperfusion; animals were killed 3, 8, or 24 hours after reperfusion. The effect of captopril or losartan (100 or 5 mg/kg intragastrically, respectively) was compared with that of vehicle (saline). The expression of angiotensinogen in liver increased fivefold 3 hours after reperfusion. Indices of liver damage and inflammation (e.g., alanine aminotransferase levels, pathological features, tumor necrosis factor‐α levels, and intercellular adhesion molecule‐1 expression) all were significantly elevated in vehicle‐treated animals after hepatic ischemia and subsequent reperfusion. Ischemia and reperfusion also caused an increase in the accumulation of protein adducts of 4‐hydroxynonenal, an index of oxidative stress. Captopril or losartan treatment showed profound protective effects under these conditions, significantly blunting the increase in all these parameters caused by ischemia and reperfusion. In conclusion, RAS inhibitors prevent acute liver injury in a model of inflammation caused by ischemia and reperfusion. These data further suggest that the RAS may play a key role in mediating such responses in the liver and suggest a novel role for this system. (HEPATOLOGY 2004;40:583–589.)

Fibrin accumulation plays a critical role in the sensitization to lipopolysaccharide‐induced liver injury caused by ethanol in mice

The early stages of alcohol‐induced liver injury involve chronic inflammation. Whereas mechanisms by which this effect is mediated are not completely understood, it is hypothesized that enhanced sensitivity to circulating lipopolysaccharide (LPS) contributes to this process. It has recently been shown that ethanol induces activation of plasminogen activator inhibitor‐1 (PAI‐1). PAI‐1 causes fibrin accumulation in liver by inhibiting degradation of fibrin (fibrinolysis). LPS also enhances fibrin accumulation by activating the coagulation cascade. It was therefore hypothesized that ethanol will synergistically increase fibrin accumulation caused by LPS, enhancing liver damage. Accordingly, the effect of ethanol pretreatment on LPS‐induced liver injury and fibrin deposition was determined in mice. Ethanol enhanced liver damage caused by LPS, as determined by plasma parameters and histological indices of inflammation and damage. This effect was concomitant with a significant increase in PAI‐1 expression. Extracellular fibrin accumulation caused by LPS was also robustly increased by ethanol preexposure. Coadministration of the thrombin inhibitor hirudin or the MEK (mitogen‐activated protein kinase) inhibitor U0126 significantly attenuated the enhanced liver damage caused by ethanol preexposure; this protection correlated with a significant blunting of the induction of PAI‐1 caused by ethanol/LPS. Furthermore, thrombin/MEK inhibition prevented the synergistic effect of ethanol on the extracellular accumulation of fibrin caused by LPS. Similar protective effects on fibrin accumulation were observed in tumor necrosis factor receptor 1 (TNFR‐1)−/− mice or in wild‐type injected with PAI‐1‐inactivating antibody. Conclusion: These results suggest that enhanced LPS‐induced liver injury caused by ethanol is mediated, at least in part, by fibrin accumulation in livers, mediated by an inhibition of fibrinolysis by PAI‐1. These results also support the hypothesis that fibrin accumulation may play a critical role in the development of early alcohol‐induced liver injury. (HEPATOLOGY 2009.)

PAI-1 plays a protective role in CCl4-induced hepatic fibrosis in mice: role of hepatocyte division

Plasminogen activator inhibitor-1 (PAI-1) is an acute phase protein that has been shown to play a role in experimental fibrosis caused by bile duct ligation (BDL) in mice. However, its role in more severe models of hepatic fibrosis (e.g., carbon tetrachloride; CCl(4)) has not been determined and is important for extrapolation to human disease. Wild-type or PAI-1 knockout mice were administered CCl(4) (1 ml/kg body wt ip) 2x/wk for 4 wk. Plasma (e.g., transaminase activity) and histological (e.g., Sirius red staining) indexes of liver damage and fibrosis were evaluated. Proliferation and apoptosis were assessed by PCNA and TdT-mediated dUTP nick-end labeling (TUNEL) staining, respectively, as well as by indexes of cell cycle (e.g., p53, cyclin D1). In contrast to previous studies with BDL, hepatic fibrosis was enhanced in PAI-1(-/-) mice after chronic CCl(4) administration. Indeed, all indexes of liver damage were elevated in PAI-1(-/-) mice compared with wild-type mice. This enhanced liver damage correlated with impaired hepatocyte proliferation. A similar effect on proliferation was observed after one bolus dose of CCl(4), without concomitant increases in liver damage. Under these conditions, a decrease in phospho-p38, coupled with elevated p53 protein, was observed; these results suggest impaired proliferation and a potential G(1)/S cell cycle arrest in PAI-1(-/-) mice. These data suggest that PAI-1 may play multiple roles in chronic liver diseases, both protective and damaging, the latter mediated by its influence on inflammation and fibrosis and the former via helping maintain hepatocyte division after an injury.

New Role of Resistin in Lipopolysaccharide-Induced Liver Damage in Mice

Studies in rodents suggest that the adipocytokine resistin causes insulin resistance via impairing normal insulin signaling. However, in humans, resistin may play a more important role in inflammation than in insulin resistance. Whether resistin contributes to inflammation in rodents is unclear. Therefore, the purpose of the present study was to determine the effect of resistin exposure on the basal and stimulated [lipopolysaccharide (LPS)] inflammatory response in mouse liver in vivo. Resistin alone had no major effects on hepatic expression of insulin-responsive genes, either in the presence or absence of LPS. Although it had no effect alone, resistin significantly enhanced hepatic inflammation and necrosis caused by LPS. Resistin increased expression of proinflammatory genes, e.g., plasminogen activator inhibitor (PAI)-1, and activity of mitogen-activated protein (MAP) kinase, extracellular signal-regulated kinase 1/2, caused by LPS, but had little effect on anti-inflammatory gene expression. Resistin also enhanced fibrin deposition (an index of hemostasis) caused by LPS. The increase in PAI-1 expression, fibrin deposition, and liver damage caused by LPS + resistin was almost completely prevented either by inhibiting the coagulation cascade, hirudin, or by blocking MAP kinase signaling, U0126 [1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio) butadiene], indicating that these pathways play a causal role in observed enhanced liver damage caused by resistin. Taken together, the augmentation of LPS-induced liver damage caused by resistin seems to involve, at least in part, up-regulation of hepatic inflammation via mechanisms most likely involving the coagulation cascade and fibrin accumulation. These data also suggest that resistin may have proinflammatory roles in mouse liver independent of its effects on insulin signaling, analogous to previous work in humans.

Contribution of the sympathetic hormone epinephrine to the sensitizing effect of ethanol on LPS-induced liver damage in mice

It is well known that ethanol preexposure sensitizes the liver to LPS hepatotoxicity. The mechanisms by which ethanol enhances LPS-induced liver injury are not completely elucidated but are known to involve an enhanced inflammatory response. Ethanol exposure also increases the metabolic rate of the liver, and this effect of ethanol on liver is mediated, at least in part, by the sympathetic hormone, epinephrine. However, whether or not the sympathetic nervous system also contributes to the sensitizing effect of ethanol preexposure on LPS-induced liver damage has not been determined. The purpose of this study was therefore to test the hypotheses that 1) epinephrine preexposure enhances LPS-induced liver damage (comparable to that of ethanol preexposure) and that 2) the sympathetic nervous system contributes to the sensitizing effect of ethanol. Accordingly, male C57BL/6J mice were administered epinephrine for 5 days (2 mg/kg per day) via osmotic pumps or bolus ethanol for 3 days (6 g/kg per day) by gavage. Twenty-four hours later, mice were injected with LPS (10 mg/kg ip). Both epinephrine and ethanol preexposure exacerbated LPS-induced liver damage and inflammation. Concomitant administration of propranolol with ethanol significantly attenuated the sensitizing effect of ethanol on LPS-induced liver damage. These data support the hypothesis that the sympathetic nervous system contributes, at least in part, to the mechanism of the sensitizing effect of ethanol. These results also suggest that sympathetic tone may contribute to the initiation and progression of alcoholic liver disease.

PKCε plays a causal role in acute ethanol-induced steatosis

Steatosis is a critical stage in the pathology of alcoholic liver disease (ALD), and preventing steatosis could protect against later stages of ALD. PKCepsilon has been shown to contribute to hepatic steatosis in experimental non-alcoholic fatty liver disease (NAFLD); however, the role of PKCepsilon in ethanol-induced steatosis has not been determined. The purpose of this study was to therefore test the hypothesis that PKCepsilon contributes to ethanol-induced steatosis. Accordingly, the effect of acute ethanol on indices of hepatic steatosis and insulin signaling were determined in PKCepsilon knockout mice and in wild-type mice that received an anti-sense oligonucleotide (ASO) to knockdown PKCepsilon expression. Acute ethanol (6g/kg i.g.) caused a robust increase in hepatic non-esterified free fatty acids (NEFA), which peaked 1h after ethanol exposure. This increase in NEFA was followed by elevated diacylglycerols (DAG), as well as by the concomitant activation of PKCepsilon. Acute ethanol also changed the expression of insulin-responsive genes (i.e. increased G6Pase, downregulated GK), in a pattern indicative of impaired insulin signaling. Acute ethanol exposure subsequently caused a robust increase in hepatic triglycerides. The accumulation of triglycerides caused by ethanol was blunted in ASO-treated or in PKCepsilon(-/-) mice. Taken together, these data suggest that the increase in NEFA caused by hepatic ethanol metabolism leads to an increase in DAG production via the triacylglycerol pathway. DAG then subsequently activates PKCepsilon, which then exacerbates hepatic lipid accumulation by inducing insulin resistance. These data also suggest that PKCepsilon plays a causal role in at least the early phases of ethanol-induced liver injury.