Reduced oncotic necrosis in Fas receptor-deficient C57BL/6J-lpr mice after bile duct ligation

Cholangiocarcinoma

Cholangiocarcinoma (CC) is a heterogeneous group of malignancies that originates at any point along the biliary tree. CC is an uncommon malignancy as it represents approximately 3% of all gastrointestinal malignancies, though its global incidence is rising. CC can often be asymptomatic in its early stages and as a result, it is frequently diagnosed in later stages, leading to challenges in clinical management.

Liver's influence on the brain through the action of bile acids

The liver partakes as a sensor and effector of peripheral metabolic changes and a regulator of systemic blood and nutrient circulation. As such, abnormalities arising from liver dysfunction can influence the brain in multiple ways, owing to direct and indirect bilateral communication between the liver and the brain. Interestingly, altered bile acid composition resulting from perturbed liver cholesterol metabolism influences systemic inflammatory responses, blood-brain barrier permeability, and neuron synaptic functions. Furthermore, bile acids produced by specific bacterial species may provide a causal link between dysregulated gut flora and neurodegenerative disease pathology through the gut-brain axis. This review will cover the role of bile acids-an often-overlooked category of active metabolites-in the development of neurological disorders associated with neurodegeneration. Further studies into bile acid signaling in the brain may provide insights into novel treatments against neurological disorders.

Role of bile acids in inflammatory liver diseases

Bile acids and their signaling pathways are increasingly recognized as potential therapeutic targets for cholestatic and metabolic liver diseases. This review summarizes new insights in bile acid physiology, focusing on regulatory roles of bile acids in the control of immune regulation and on effects of pharmacological modulators of bile acid signaling pathways in human liver disease. Recent mouse studies have highlighted the importance of the interactions between bile acids and gut microbiome. Interfering with microbiome composition may be beneficial for cholestatic and metabolic liver diseases by modulating formation of secondary bile acids, as different bile acid species have different signaling functions. Bile acid receptors such as FXR, VDR, and TGR5 are expressed in a variety of cells involved in innate as well as adaptive immunity, and specific microbial bile acid metabolites positively modulate immune responses of the host. Identification of Cyp2c70 as the enzyme responsible for the generation of hydrophilic mouse/rat-specific muricholic acids has allowed the generation of murine models with a human-like bile acid composition. These novel mouse models will aid to accelerate translational research on the (patho)physiological roles of bile acids in human liver diseases .

Necroptosis in Cholangiocarcinoma

Necroptosis is a type of regulated cell death that is increasingly being recognized as a relevant pathway in different pathological conditions. Necroptosis can occur in response to multiple stimuli, is triggered by the activation of death receptors, and is regulated by receptor-interacting protein kinases 1 and 3 and mixed-lineage kinase domain-like, which form a regulatory complex called the necrosome. Accumulating evidence suggests that necroptosis plays a complex role in cancer, which is likely context-dependent and can vary among different types of neoplasms. Necroptosis serves as an alternative mode of programmed cell death overcoming apoptosis and, as a pro-inflammatory death type, it may inhibit tumor progression by releasing damage-associated molecular patterns to elicit robust cross-priming of anti-tumor CD8+ T cells. The development of therapeutic strategies triggering necroptosis shows great potential for anti-cancer therapy. In this review, we summarize the current knowledge on necroptosis and its role in liver biliary neoplasms, underlying the potential of targeting necroptosis components for cancer treatment.

Inflammation and Cell Death During Cholestasis: The Evolving Role of Bile Acids

Cholestasis results in blockage of bile flow whether the point of obstruction occurs extrahepatically or intrahepatically. Bile acids are a primary constituent of bile, and thus one of the primary outcomes is acute retention of bile acids in hepatocytes. Bile acids are normally secreted into the biliary tracts and then released into the small bowel before recirculating back to the liver. Retention of bile acids has long been hypothesized to be a primary cause of the associated liver injury that occurs during acute or chronic cholestasis. Despite this, a surge of papers in the last decade have reported a primary role for inflammation in the pathophysiology of cholestatic liver injury. Furthermore, it has increasingly been recognized that both the constituency of individual bile acids that make up the greater pool, as well as their conjugation status, is intimately involved in their toxicity, and this varies between species. Finally, the role of bile acids in drug-induced cholestatic liver injury remains an area of increasing interest. The purpose of this review is to critically evaluate current proposed mechanisms of cholestatic liver injury, with a focus on the evolving role of bile acids in cell death and inflammation.

Direct Amplification of Tissue Factor:Factor VIIa Procoagulant Activity by Bile Acids Drives Intrahepatic Coagulation

OBJECTIVE

Regulation of TF (tissue factor):FVIIa (coagulation factor VIIa) complex procoagulant activity is especially critical in tissues where plasma can contact TF-expressing cells. One example is the liver, where hepatocytes are routinely exposed to plasma because of the fenestrated sinusoidal endothelium. Although liver-associated TF contributes to coagulation, the mechanisms controlling the TF:FVIIa complex activity in this tissue are not known. Approach and Results: Common bile duct ligation in mice triggered rapid hepatocyte TF-dependent intrahepatic coagulation coincident with increased plasma bile acids, which occurred at a time before observable liver damage. Similarly, plasma TAT (thrombin-antithrombin) levels increased in cholestatic patients without concurrent hepatocellular injury. Pathologically relevant concentrations of the bile acid glycochenodeoxycholic acid rapidly increased hepatocyte TF-dependent procoagulant activity in vitro, independent of de novo TF synthesis and necrotic or apoptotic cell death. Glycochenodeoxycholic acid increased hepatocyte TF activity even in the presence of the phosphatidylserine-blocking protein lactadherin. Interestingly, glycochenodeoxycholic acid and taurochenodeoxycholic acid increased the procoagulant activity of the TF:FVIIa complex relipidated in unilamellar phosphatidylcholine vesicles, which was linked to an apparent decrease in the K_m for FX (coagulation factor X). Notably, the zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate, a bile acid structural analog, did not increase relipidated TF:FVIIa activity. Bile acids directly enhanced factor X activation by recombinant soluble TF:FVIIa complex but had no effect on FVIIa alone.

CONCLUSIONS

The results indicate that bile acids directly accelerate TF:FVIIa-driven coagulation reactions, suggesting a novel mechanism whereby elevation in a physiological mediator can directly increase TF:FVIIa procoagulant activity.

Disruption of the FasL/Fas axis protects against inflammation-derived tumorigenesis in chronic liver disease

Fas Ligand (FasL) and Fas (APO-1/CD95) are members of the TNFR superfamily and may trigger apoptosis. Here, we aimed to elucidate the functional role of Fas signaling in an experimental model of chronic liver disease, the hepatocyte-specific NEMO knockout (NEMO^Δhepa) mice. We generated NEMO^Δhepa /Fas^lpr mice, while NEMO^Δhepa, NEMO^f/f as well as Fas^lpranimals were used as controls, and characterized their phenotype during liver disease progression. Liver damage was evaluated by serum transaminases, histological, immunofluorescence procedures, and biochemical and molecular biology techniques. Proteins were detected by western Blot, expression of mRNA by RT-PCR, and infiltration of inflammatory cells was determined by FACs analysis, respectively. Fas^lpr mutation in NEMO^Δhepa mice resulted in overall decreased liver injury, enhanced hepatocyte survival, and reduced proliferation at 8 weeks of age compared with NEMO^Δhepa mice. Moreover, NEMO^Δhepa/Fas^lpr animals elicited significantly decreased parameters of liver fibrosis, such as Collagen IA1, MMP2, and TIMP1, and reduced proinflammatory macrophages and cytokine expression. At 52 weeks of age, NEMO^Δhepa/Fas^lpr exhibited less malignant growth as evidenced by reduced HCC burden associated with a significantly decreased number of nodules and LW/BW ratio and decreased myeloid populations. Deletion of TNFR1 further reduced tumor load of 52-weeks-old NEMO^Δhepa/Fas^lpr mice. The functionality of FasL/Fas might affect inflammation-driven tumorigenesis in an experimental model of chronic liver disease. These results help to develop alternative therapeutic approaches and extend the limitations of tumor therapy against HCC.

Alcoholic Hepatitis: Lost in Translation

Alcoholic hepatitis is the most severe and acute form of alcoholic liver disease. The mortality rate associated with alcoholic hepatitis is high, largely due to the lack of suitable pharmacological interventions. While there has been substantial research in the area, generating pharmacological interventions has been plagued by the lack of a robust mouse model both for testing and for understanding the underlying pathology. A number of major notable advances have been made in this area recently, with the goal of generating a mouse model of alcoholic hepatitis. The purpose of this article is to review recent advances in modeling alcoholic liver disease both in vitro and in vivo in the mouse, and place them in the context of the greater spectrum of alcoholic liver disease, with a focus on how we can translate current advances into a high-fidelity model of alcoholic hepatitis. In addition, we will review the basic mechanisms of alcoholic hepatitis as it is currently understood, focusing on recent advancements in diagnosis, prognosis and current pathophysiology, especially as it relates to the profound immune dysfunction present during alcoholic hepatitis.

Cell Death and Prognosis of Mortality in Alcoholic Hepatitis Patients Using Plasma Keratin-18

Alcoholic liver disease encompasses the progressive stages of liver dysfunction that culminates in alcoholic cirrhosis (AC) and in severe cases alcoholic hepatitis (AH). Currently, prognostic scores have limited specificity and sensitivity. Plasma keratin-18 (K18) levels are elevated during liver disease and may be biomarkers of outcome. The objective of this study was to determine if total K18 (M65) or caspase-cleaved K18 (M30) levels were different between AC and AH patients. M65 and M30 levels were measured in the plasma of consented healthy controls and patients with AC and AH. Cell death was assessed by TUNEL staining and caspase activity. M65 and M30 values were significantly higher in AC patients compared to healthy controls and further increased in AH patients. The M65 values and the M30/M65 ratios of nonsurviving AH patients were significantly elevated above their surviving counterparts and healthy controls. Statistical analysis indicated that M30/M65 ratios outperformed current indices for accurately distinguishing the prognosis of AH patients. These scores occurred with minimal increase in plasma cell death markers such as ALT and AST. Serum caspase activity, TUNEL staining, and M30 immunohistochemistry in biopsies indicated that serum and tissue values may not correlate well with overall cell death. In conclusion, both M65 and M30 differentiate AH from AC patients, and M65 values and the M30/M65 ratio are capable of predicting early stage mortality; however, they may not accurately reflect pure hepatocyte cell death in these populations, as they do not strongly correlate with traditional cell death markers.

Possible Role of Phosphatidylcholine and Sphingomyelin on Fumonisin B1-mediated Toxicity

A major corn-related mycotoxin, fumonisin B1 (FB1), continues to attract attention of researchers as well as risk-assessors due to the diverse toxicological characteristics, including distinct target tissues in different animal species and opposite susceptibility in males and females in mice and rats. More than thirty years passed since the structure identification as a sphingoid-like chemical, but the causal mechanism of the toxicity remains obscure in spites of extensive studies. Considerable amounts of knowledge have been accumulated on the biochemical/toxicological actions of FB1, but the influence on lipid dynamics and mobilization in the body has not been focused well in relation to the FB1-mediated toxicity. Considerable influences of this toxin on mobilization of sphingolipids and phospholipids and also on adaptive changes in their compositions in tissues are implicated from recent studies on FB1-interacting ceramide synthases. Accumulated patho-physiological data also suggest a possible role of hepatic phospholipid on FB1-mediated toxicity. Thus, a mechanism of FB1-mediated toxicity is discussed in relation to the mobilization of phospholipids and sphingolipids in the body in this context.

The impact of sterile inflammation in acute liver injury

BACKGROUND

The liver has a number of functions in innate immunity. These functions predispose the liver to innate immune-mediated liver injury when inflammation goes unchecked. Significant progress has been made in the last 25 years on sterile inflammatory liver injury in a number of models; however, a great deal of controversy and many questions about the nature of sterile inflammation still exist.

AIM

The goal of this article is to review sterile inflammatory liver injury using both a basic approach to what constitutes the inflammatory injury, and through examination of current models of liver injury and inflammation. This information will be tied to human patient conditions when appropriate.

RELEVANCE FOR PATIENTS

Inflammation is one of the most critical factors for managing in-patient liver disease in a number of scenarios. More information is needed for both scientists and clinicians to develop rational treatments.

The protective effect of the natural compound hesperetin against fulminant hepatitis in vivo and in vitro

BACKGROUND AND PURPOSE

Liver diseases are mostly accompanied by inflammation and hepatocyte death. Therapeutic approaches targeting both hepatocyte injury and inflammation are not available. Natural compounds are considered as potential treatment for inflammatory liver diseases. Hesperetin, a flavonoid component of citrus fruits, has been reported to have anti-inflammatory properties. The aim of this study was to evaluate the cytoprotective and anti-inflammatory properties of hesperetin both in vitro and in models of fulminant hepatitis.

EXPERIMENTAL APPROACH

Apoptotic cell death and inflammation were induced in primary cultures of rat hepatocytes by bile acids and cytokine mixture respectively. Apoptosis was quantified by caspase-3 activity and necrosis by LDH release. The concanavalin A (ConA) and D-galactosamine/LPS (D-GalN/LPS) were used as models of fulminant hepatitis. Liver injury was assessed by alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels, liver histology and TUNEL assay and inflammation by inducible NOS (iNOS) expression.

KEY RESULTS

Hesperetin blocked bile acid-induced apoptosis and cytokine-induced inflammation in rat hepatocytes. Moreover, hesperetin improved liver histology and protected against hepatocyte injury in ConA- and D-GalN/LPS-induced fulminant hepatitis, as assessed by TUNEL assay and serum AST and ALT levels. Hesperetin also reduced expression of the inflammatory marker iNOS and the expression and serum levels of TNFα and IFN-γ, the main mediators of cell toxicity in fulminant hepatitis.

CONCLUSION AND IMPLICATIONS

Hesperetin has anti-inflammatory and cytoprotective actions in models of acute liver toxicity. Hesperetin therefore has therapeutic potential for the treatment of inflammatory liver diseases accompanied by extensive hepatocyte injury, such as fulminant hepatitis.

Critical Factors in the Assessment of Cholestatic Liver Injury In Vitro

Cholestasis is a common pathological component of numerous liver diseases. The initiating event during cholestatic liver injury is widely believed to be the accumulation of bile acids in hepatocytes and the hepatic parenchyma. As bile acids are considered the primary toxic compounds in the injury, numerous in vitro models of bile acid-induced injury and bile acid-induced changes in gene expression have been developed to attempt to better define cholestasis at a cellular level. This chapter focuses on the establishment of a system for determining the effects of cholestatic concentrations of bile acids on hepatocytes using primary hepatocytes or hepatoma cell lines. Moreover, this chapter addresses significant differences in the response of different species to bile acid exposure and novel information on the relevance of treating hepatocytes with concentrations of specific bile acids.

miR-223 Deficiency Protects against Fas-Induced Hepatocyte Apoptosis and Liver Injury through Targeting Insulin-Like Growth Factor 1 Receptor

The biological functions and molecular mechanisms of miR-223 action in liver cells and liver diseases remain unclear. We therefore determined the effect and mechanism of action of miR-233 in Fas-induced hepatocyte apoptosis and liver injury. Wild-type (WT) and miR-223 knockout (KO) mice were treated i.p. with 0.5 μg/g body weight anti-Fas antibody Jo2, and the animals were monitored for survival and the extent of liver injury. Although WT mice died 4 to 6 hours after Jo2 injection (n = 6), all of the miR-223 KO mice (n = 6) survived. In comparison to WT mice, the miR-223 KO mice showed resistance to Fas-induced liver injury, as indicated by less tissue damage under histopathological examination, fewer apoptotic hepatocytes under caspase-3 immunostaining, and less elevation of serum transaminases. miR-223 KO livers showed less caspase-3, caspase-8, and caspase-9 activation and less poly (ADP-ribose) polymerase cleavage compared with WT livers (P < 0.05). Furthermore, tail vein injection of miR-223 lentiviral vector to miR-223 KO mice restored Jo2-induced liver injury. Transfection of miR-223 KO hepatocytes with miR-223 mimic enhanced Jo2-induced activation of caspase-3, caspase-8, and caspase-9, whereas transfection of WT hepatocytes with the miR-223 inhibitor attenuated Jo2-induced apoptosis. These findings demonstrate that miR-223 deficiency protects against Fas-induced hepatocyte apoptosis and liver injury. Further in vitro and in vivo data indicate that miR-223 regulates Fas-induced hepatocyte apoptosis and liver injury by targeting the insulin-like growth factor 1 receptor.

Deletion of Mir155 prevents Fas-induced liver injury through up-regulation of Mcl-1

Fas-induced apoptosis is involved in diverse liver diseases. Herein, we investigated the effect of Mir155 deletion on Fas-induced liver injury. Wild-type (WT) mice and Mir155 knockout (KO) mice were i.p. administered with the anti-Fas antibody (Jo2) to determine animal survival and the extent of liver injury. After Jo2 injection, the Mir155 KO mice exhibited prolonged survival versus the WT mice (P < 0.01). The Mir155 KO mice showed lower alanine aminotransferase and aspartate aminotransferase levels, less liver tissue damage, fewer apoptotic hepatocytes, and lower liver tissue caspase 3/7, 8, and 9 activities compared with the WT mice, indicating that Mir155 deletion prevents Fas-induced hepatocyte apoptosis and liver injury. Hepatocytes isolated from Mir155 KO mice also showed resistance to Fas-induced apoptosis, in vitro. Higher protein level of myeloid cell leukemia-1 (Mcl-1) was also observed in Mir155 KO hepatocytes compared to WT hepatocytes. A miR-155 binding site was identified in the 3'-untranslated region of Mcl-1 mRNA; Mcl1 was identified as a direct target of miR-155 in hepatocytes. Consistently, pretreatment with a siRNA specific for Mcl1 reversed Mir155 deletion-mediated protection against Jo2-induced liver tissue damage. Finally, restoration of Mir155 expression in Mir155 KO mice abolished the protection against Fas-induced hepatocyte apoptosis. Taken together, these findings demonstrate that deletion of Mir155 prevents Fas-induced hepatocyte apoptosis and liver injury through the up-regulation of Mcl1.

Critical review of resveratrol in xenobiotic-induced hepatotoxicity

Use of natural products is increasingly popular. In fact, many patients with liver diseases self-medicate with herbal supplements. Resveratrol (RSV), in particular, is a common natural product that can reduce injury in experimental models of liver disease. Xenobiotic hepatotoxicity is a particularly important area-of-need for therapeutics. Drug-induced liver injury, for example, is the most common cause of acute liver failure (ALF) and ALF-induced deaths in many countries. Importantly, RSV protects against hepatotoxicity in animal models in vivo caused by several drugs and chemicals and may be an effective intervention. Although many mechanisms have been proposed to explain the protection, not all are consistent with other data. Furthermore, RSV suffers from other issues, including limited bioavailability due to extensive hepatic metabolism. The purpose of this article is to summarize recent findings on the protective effects of RSV in xenobiotic-induced liver injury and other forms of liver injury and to provide a critical review of the underlying mechanisms. New mechanisms that are more consistent with data emerging from the toxicology field are suggested. Efforts to move RSV into clinical use are also considered. Overall, RSV is a promising candidate for therapeutic use, but additional studies are needed to better understand its effects.

Therapeutic targets for cholestatic liver injury

INTRODUCTION

Cholestasis is a reduction in bile flow that occurs during numerous pathologies. Blockage of the biliary tracts results in hepatic accumulation of bile acids or their conjugate bile salts. The molecular mechanisms behind liver injury associated with cholestasis are extensively studied, but not well understood. Multiple models of obstructive cholestasis result in a significant inflammatory infiltrate at the sites of necrosis that characterize the injury.

AREAS COVERED

This review will focus on direct bile acid toxicity during cholestasis, bile acid signaling processes and on the development and continuation of inflammation during cholestasis, with a focus on novel proposed molecular mediators of neutrophil recruitment. While significant progress has been made on these molecular mechanisms, a continued focus on how cholestasis and the innate immune system interact is necessary to discover targetable therapeutics that might protect the liver while leaving global immunity intact.

EXPERT OPINION

While bile acid toxicity likely occurs in humans and other mammals when toxic bile acids accumulate, persistent inflammation is likely responsible for continued liver injury during obstructive cholestasis. Targeting molecular mediators of inflammation may help prevent liver injury during acute cholestasis both in murine models and human patients.

Contrast-enhanced micro-computed tomography using ExiTron nano6000 for assessment of liver injury

AIM: To explore the potential of contrast-enhanced computed tomography (CECT) using ExiTron nano6000 for assessment of liver lesions in mouse models.

METHODS: Three mouse models of liver lesions were used: bile duct ligation (BDL), lipopolysaccharide (LPS)/D-galactosamine (D-GalN), and alcohol. After injection with the contrast agent ExiTron nano6000, the mice were scanned with micro-CT. Liver lesions were evaluated using CECT images, hematoxylin and eosin staining, and serum aminotransferase levels. Macrophage distribution in the injury models was shown by immunohistochemical staining of CD68. The in vitro studies measured the densities of RAW264.7 under different conditions by CECT.

RESULTS: In the in vitro studies, CECT provided specific and strong contrast enhancement of liver in mice. CECT could present heterogeneous images and densities of injured livers induced by BDL, LPS/D-GalN, and alcohol. The liver histology and immunochemistry of CD68 demonstrated that both dilated biliary tracts and necrosis in the injured livers could lead to the heterogeneous distribution of macrophages. The in vitro study showed that the RAW264.7 cell masses had higher densities after LPS activation.

CONCLUSION: Micro-CT with the contrast agent ExiTron nano6000 is feasible for detecting various liver lesions by emphasizing the heterogeneous textures and densities of CECT images.

Bile acid-induced necrosis in primary human hepatocytes and in patients with obstructive cholestasis

Accumulation of bile acids is a major mediator of cholestatic liver injury. Recent studies indicate bile acid composition between humans and rodents is dramatically different, as humans have a higher percent of glycine conjugated bile acids and increased chenodeoxycholate content, which increases the hydrophobicity index of bile acids. This increase may lead to direct toxicity that kills hepatocytes, and promotes inflammation. To address this issue, this study assessed how pathophysiological concentrations of bile acids measured in cholestatic patients affected primary human hepatocytes. Individual bile acid levels were determined in serum and bile by UPLC/QTOFMS in patients with extrahepatic cholestasis with, or without, concurrent increases in serum transaminases. Bile acid levels increased in serum of patients with liver injury, while biliary levels decreased, implicating infarction of the biliary tracts. To assess bile acid-induced toxicity in man, primary human hepatocytes were treated with relevant concentrations, derived from patient data, of the model bile acid glycochenodeoxycholic acid (GCDC). Treatment with GCDC resulted in necrosis with no increase in apoptotic parameters. This was recapitulated by treatment with biliary bile acid concentrations, but not serum concentrations. Marked elevations in serum full-length cytokeratin-18, high mobility group box 1 protein (HMGB1), and acetylated HMGB1 confirmed inflammatory necrosis in injured patients; only modest elevations in caspase-cleaved cytokeratin-18 were observed. These data suggest human hepatocytes are more resistant to human-relevant bile acids than rodent hepatocytes, and die through necrosis when exposed to bile acids. These mechanisms of cholestasis in humans are fundamentally different to mechanisms observed in rodent models.

Decreasing mitochondrial fission prevents cholestatic liver injury

Mitochondria frequently change their shape through fission and fusion in response to physiological stimuli as well as pathological insults. Disrupted mitochondrial morphology has been observed in cholestatic liver disease. However, the role of mitochondrial shape change in cholestasis is not defined. In this study, using in vitro and in vivo models of bile acid-induced liver injury, we investigated the contribution of mitochondrial morphology to the pathogenesis of cholestatic liver disease. We found that the toxic bile salt glycochenodeoxycholate (GCDC) rapidly fragmented mitochondria, both in primary mouse hepatocytes and in the bile transporter-expressing hepatic cell line McNtcp.24, leading to a significant increase in cell death. GCDC-induced mitochondrial fragmentation was associated with an increase in reactive oxygen species (ROS) levels. We found that preventing mitochondrial fragmentation in GCDC by inhibiting mitochondrial fission significantly decreased not only ROS levels but also cell death. We also induced cholestasis in mouse livers via common bile duct ligation. Using a transgenic mouse model inducibly expressing a dominant-negative fission mutant specifically in the liver, we demonstrated that decreasing mitochondrial fission substantially diminished ROS levels, liver injury, and fibrosis under cholestatic conditions. Taken together, our results provide new evidence that controlling mitochondrial fission is an effective strategy for ameliorating cholestatic liver injury.

Lithocholic acid feeding results in direct hepato-toxicity independent of neutrophil function in mice

Lithocholic acid (LCA) supplementation in the diet results in intrahepatic cholestasis and bile infarcts. Previously we showed that an innate immune response is critical for cholestatic liver injury in the bile duct ligated mice. Thus, the purpose of this study was to investigate the role of neutrophils in the mechanism of liver injury caused by feeding mice a diet containing LCA. C57BL/6 mice were given control or 1% LCA containing diet for 24-96 h and then examined for parameters of hepatotoxicity. Plasma ALT levels were significantly increased by 48 h after LCA feeding, which correlated with both neutrophil recruitment to the liver and upregulation of numerous pro-inflammatory genes. The injury was confirmed by histology. Deficiency in intercellular adhesion molecule-1 (ICAM-1) expression or inhibition of neutrophil function failed to protect against the injury. Bile acid levels were quantified in plasma and bile of LCA-fed mice after 48 and 96 h. Only the observed biliary levels of taurochenodeoxycholic acid and potentially tauro-LCA caused direct cytotoxicity in mouse hepatocytes. These data support the conclusion that neutrophil recruitment occurs after the onset of bile acid-induced necrosis in LCA-fed animals, and is not a primary mechanism of cell death when cholestasis occurs through accumulation of hydrophobic bile acids.

Apoptosis and necrosis in the liver

Because of its unique function and anatomical location, the liver is exposed to a multitude of toxins and xenobiotics, including medications and alcohol, as well as to infection by hepatotropic viruses, and therefore, is highly susceptible to tissue injury. Cell death in the liver occurs mainly by apoptosis or necrosis, with apoptosis also being the physiologic route to eliminate damaged or infected cells and to maintain tissue homeostasis. Liver cells, especially hepatocytes and cholangiocytes, are particularly susceptible to death receptor-mediated apoptosis, given the ubiquitous expression of the death receptors in the organ. In a quite unique way, death receptor-induced apoptosis in these cells is mediated by both mitochondrial and lysosomal permeabilization. Signaling between the endoplasmic reticulum and the mitochondria promotes hepatocyte apoptosis in response to excessive free fatty acid generation during the metabolic syndrome. These cell death pathways are partially regulated by microRNAs. Necrosis in the liver is generally associated with acute injury (i.e., ischemia/reperfusion injury) and has been long considered an unregulated process. Recently, a new form of "programmed" necrosis (named necroptosis) has been described: the role of necroptosis in the liver has yet to be explored. However, the minimal expression of a key player in this process in the liver suggests this form of cell death may be uncommon in liver diseases. Because apoptosis is a key feature of so many diseases of the liver, therapeutic modulation of liver cell death holds promise. An updated overview of these concepts is given in this article.

Osteopontin is an initial mediator of inflammation and liver injury during obstructive cholestasis after bile duct ligation in mice

Osteopontin (OPN) is a chemotactic factor which can be cleaved to the pro-inflammatory form by matrix metalloproteinases (MMPs). To test the hypothesis that OPN can modulate inflammatory liver injury during cholestasis, wild-type (WT) C57BL/6 and OPN knockout (OPN-KO) mice underwent bile duct ligation (BDL). OPN-KO mice showed significant reduction in liver injury (plasma ALT and necrosis) and neutrophil recruitment compared with WT animals at 24h but not 72h after BDL. In WT mice, a 4-fold increase in hepatic MMP-3 mRNA and elevated MMP activities and cleaved OPN levels were observed in bile. WT mice subjected to BDL in the presence of the MMP inhibitor BB-94 showed reduced liver injury, less neutrophil extravasation and diminished levels of cleaved OPN in bile. Thus, during obstructive cholestasis, OPN released from biliary epithelial cells could be cleaved by MMPs in bile. When the biliary system leaks, cleaved OPN enters the parenchyma and attracts neutrophils. In the absence of OPN, other chemoattractants, e.g. chemokines, mediate a delayed inflammatory response and injury. Taken together, our data suggest that OPN is the pro-inflammatory mediator that initiates the early neutrophil-mediated injury phase during obstructive cholestasis in mice.

Plasma biomarkers of liver injury and inflammation demonstrate a lack of apoptosis during obstructive cholestasis in mice

Cholestasis is a pathological common component of numerous liver diseases that results in hepatotoxicity, inflammation, and cirrhosis when untreated. While the predominant hypothesis in cholestatic liver injury remains hepatocyte apoptosis due to direct toxicity of hydrophobic bile acid exposure, recent work suggests that the injury occurs through inflammatory necrosis. In order to resolve this controversy, we used novel plasma biomarkers to assess the mechanisms of cell death during early cholestatic liver injury. C57Bl/6 mice underwent bile duct ligation (BDL) for 6-72 h, or sham operation. Another group of mice were given d-galactosamine and endotoxin as a positive control for apoptosis and inflammatory necrosis. Plasma levels of full length cytokeratin-18 (FL-K18), microRNA-122 (miR-122) and high mobility group box-1 protein (HMGB1) increased progressively after BDL with peak levels observed after 48 h. These results indicate extensive cell necrosis after BDL, which is supported by the time course of plasma alanine aminotransferase activities and histology. In contrast, plasma caspase-3 activity, cleaved caspase-3 protein and caspase-cleaved cytokeratin-18 fragments (cK18) were not elevated at any time during BDL suggesting the absence of apoptosis. In contrast, all plasma biomarkers of necrosis and apoptosis were elevated 6 h after Gal/End treatment. In addition, acetylated HMGB1, a marker for macrophage and monocyte activation, was increased as early as 12 h but mainly at 48-72 h. However, progressive neutrophil accumulation in the area of necrosis started at 6h after BDL. In conclusion, these data indicate that early cholestatic liver injury in mice is an inflammatory event, and occurs through necrosis with little evidence for apoptosis.

Fas receptor-deficient lpr mice are protected against acetaminophen hepatotoxicity due to higher glutathione synthesis and enhanced detoxification of oxidant stress

Acetaminophen (APAP) overdose is a classical model of hepatocellular necrosis; however, the involvement of the Fas receptor in the pathophysiology remains controversial. Fas receptor-deficient (lpr) and C57BL/6 mice were treated with APAP to compare the mechanisms of hepatotoxicity. Lpr mice were partially protected against APAP hepatotoxicity as indicated by reduced plasma ALT and GDH levels and liver necrosis. Hepatic Cyp2e1 protein, adduct formation and hepatic glutathione (GSH) depletion were similar, demonstrating equivalent reactive metabolite generation. There was no difference in cytokine formation or hepatic neutrophil recruitment. Interestingly, hepatic GSH recovered faster in lpr mice than in wild type animals resulting in enhanced detoxification of reactive oxygen species. Driving the increased GSH levels, mRNA induction and protein expression of glutamate-cysteine ligase (gclc) were higher in lpr mice. Inducible nitric oxide synthase (iNOS) mRNA and protein levels at 6h were significantly lower in lpr mice, which correlated with reduced nitrotyrosine staining. Heat shock protein 70 (Hsp70) mRNA levels were substantially higher in lpr mice after APAP.

CONCLUSION

Our data suggest that the faster recovery of hepatic GSH levels during oxidant stress and peroxynitrite formation, reduced iNOS expression and enhanced induction of Hsp70 attenuated the susceptibility to APAP-induced cell death in lpr mice.

Lysosomal instability and cathepsin B release during acetaminophen hepatotoxicity

Acetaminophen (APAP) overdose is currently the most frequent cause of drug-induced liver failure in the United States. Recently, it was shown that lysosomal iron translocates to mitochondria where it contributes to the collapse of the mitochondrial membrane potential. Therefore, the purpose of this study was to investigate whether cathepsin B, a lysosomal protease, is involved in APAP-induced hepatotoxicity. Cathepsin B activity was measured in subcellular liver fractions of C57Bl/6 mice 3 hr after 300 mg/kg APAP treatment. There was a significant increase in cytoplasmic cathepsin activity, concurrent with a decrease in microsomal activity, indicative of lysosomal cathepsin B release. To investigate the effect of cathepsin B on hepatotoxicity, the cathepsin inhibitor AC-LVK-CHO was given 1 hr prior to 300 mg/kg APAP treatment along with vehicle control. There was no difference between groups in serum alanine aminotransferase (ALT) values, or by histological evaluation of necrosis, although cathepsin B activity was inhibited by 70-80% compared with controls. These findings were confirmed with a different inhibitor (z-FA-fmk) in vivo and in vitro. Hepatocytes were exposed to 5 mM acetaminophen. Lysotracker staining confirmed lysosomal instability and cathepsin B release, but there was no reduction in cell death after treatment with cathepsin B inhibitors. Finally, cathepsin B release was measured in clinical samples from patients with APAP-induced liver injury. Low levels of cathepsin B were released into plasma from overdose patients. APAP overdose causes lysosomal instability and release of cathepsin B into the cytosol but does not contribute to liver injury under these conditions.

Novel insight into mechanisms of cholestatic liver injury

Cholestasis results in a buildup of bile acids in serum and in hepatocytes. Early studies into the mechanisms of cholestatic liver injury strongly implicated bile acid-induced apoptosis as the major cause of hepatocellular injury. Recent work has focused both on the role of bile acids in cell signaling as well as the role of sterile inflammation in the pathophysiology. Advances in modern analytical methodology have allowed for more accurate measuring of bile acid concentrations in serum, liver, and bile to very low levels of detection. Interestingly, toxic bile acid levels are seemingly far lower than previously hypothesized. The initial hypothesis has been based largely upon the exposure of μmol/L concentrations of toxic bile acids and bile salts to primary hepatocytes in cell culture, the possibility that in vivo bile acid concentrations may be far lower than the observed in vitro toxicity has far reaching implications in the mechanism of injury. This review will focus on both how different bile acids and different bile acid concentrations can affect hepatocytes during cholestasis, and additionally provide insight into how these data support recent hypotheses that cholestatic liver injury may not occur through direct bile acid-induced apoptosis, but may involve largely inflammatory cell-mediated liver cell necrosis.

Bile acids induce inflammatory genes in hepatocytes: a novel mechanism of inflammation during obstructive cholestasis

Inflammation contributes to liver injury during cholestasis. The mechanism by which cholestasis initiates an inflammatory response in the liver, however, is not known. Two hypotheses were investigated in the present studies. First, activation of Toll-like receptor 4 (TLR4), either by bacterial lipopolysaccharide or by damage-associated molecular pattern molecules released from dead hepatocytes, triggers an inflammatory response. Second, bile acids act as inflammagens, and directly activate signaling pathways in hepatocytes that stimulate production of proinflammatory mediators. Liver inflammation was not affected in lipopolysaccharide-resistant C3H/HeJ mice after bile duct ligation, indicating that Toll-like receptor 4 is not required for initiation of inflammation. Treatment of hepatocytes with bile acids did not directly cause cell toxicity but increased the expression of numerous proinflammatory mediators, including cytokines, chemokines, adhesion molecules, and other proteins that influence immune cell levels and function. Up-regulation of several of these genes in hepatocytes and in the liver after bile duct ligation required early growth response factor-1, but not farnesoid X receptor. In addition, early growth response factor-1 was up-regulated in the livers of patients with cholestasis and correlated with levels of inflammatory mediators. These data demonstrate that Toll-like receptor 4 is not required for the initiation of acute inflammation during cholestasis. In contrast, bile acids directly activate a signaling network in hepatocytes that promotes hepatic inflammation during cholestasis.

The BH3-only protein bid does not mediate death-receptor-induced liver injury in obstructive cholestasis

The accumulation of bile acids during obstructive cholestasis causes liver injury and fibrosis, which is at least partly mediated by the death receptors Tumor necrosis factor-related apoptosis-inducing ligand, Tumor necrosis factor-alpha, and Fas. The BH3-interacting domain death agonist Bid is a critical mediator of death receptor-induced apoptosis in hepatocytes. Our aim for this study was, therefore, to elucidate whether Bid also mediates death receptor-induced liver injury in obstructive cholestasis. Overall, survival and various aspects of liver injury were analyzed in wild-type and Bid(-/-) mice after bile duct ligation (BDL), a commonly used model to study obstructive cholestasis in mice. Liver injury was examined at 3, 7, and 14 days after BDL. Loss of Bid did not affect the number of bile infarcts, serum aspartate aminotransferase values, or animal survival. Processing of procaspase-3 and procaspase-9, and caspase-3 enzyme activities, were not detectable in either group, and Bid(-/-) mice displayed the same pattern of terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling positive hepatocytes as wild-type controls following BDL. In contrast to Fas-receptor deficient lpr mice, hepatic fibrosis and the inflammatory response was not affected by loss of Bid. Together, these data suggest that Bid is not a downstream target of death receptors in obstructive cholestasis and does not significantly contribute to bile acid induced liver injury and fibrosis.

NIM811 (N-methyl-4-isoleucine cyclosporine), a mitochondrial permeability transition inhibitor, attenuates cholestatic liver injury but not fibrosis in mice

Cholestasis causes hepatocyte death, possibly because of mitochondrial injury. This study investigated whether NIM811 (N-methyl-4-isoleucine cyclosporine), an inhibitor of the mitochondrial permeability transition (MPT), attenuates cholestatic liver injury in vivo. Cholestasis was induced in mice by bile duct ligation (BDL). NIM811 was gavaged (20 mg/kg) before BDL and daily (10 mg/kg) afterward. Mitochondrial depolarization, cell death, and MPT onset were assessed by intravital confocal/multiphoton microscopy of rhodamine 123, propidium iodide, and calcein. After BDL, serum alanine aminotransferase (ALT), hepatic necrosis, and apoptosis all increased. NIM811 decreased ALT, necrosis, and apoptosis by 60 to 86%. In vehicle-treated mice at 6 h after BDL, viable hepatocytes with depolarized mitochondria were 18/high-power field (hpf), and nonviable cells were approximately 1/hpf, showing that depolarization preceded necrosis. Calcein entered mitochondria after BDL, indicating MPT onset in vivo. NIM811 decreased depolarization by 72%, prevented calcein entry into mitochondria, and blocked release of cytochrome c. Hepatic tumor necrosis factor alpha, transforming growth factor-beta1, procollagen alpha1(I) mRNA, alpha-smooth muscle actin, and Sirius red staining for collagen increased after BDL but were not different in vehicle- and NIM811-treated mice. Taken together, NIM811 decreased cholestatic necrosis and apoptosis but did not block fibrosis, indicating that the MPT plays an important role in cholestatic cell death in vivo.

Role of neutrophils in acute inflammatory liver injury

Hepatic infiltration of polymorphonuclear leukocytes (neutrophils) is an early response to tissue injury, cellular stress or systemic inflammation. Neutrophil activation is vital for host-defense and the removal of cell debris but can also cause additional tissue damage or even liver failure. In order to prevent the detrimental effects of neutrophils without compromising host-defense reactions, it is important to understand the mechanisms of neutrophil hepatotoxicity. The first step in the pathophysiology is the priming and recruitment of neutrophils into the liver vasculature by inflammatory mediators, e.g. cytokines, chemokines, or complement factors. Most critical for parenchymal cell damage is the accumulation in sinusoids, which does not depend on cellular adhesion molecules. The next step is the extravasation into the parenchyma. This process requires a chemotactic signal from hepatocytes or already extravasated neutrophils and depends on cellular adhesion molecules on neutrophils (beta(2) or beta(1) integrins) and on endothelial cells (intercellular or vascular cell adhesion molecules). The third step is the direct contact with target cells (hepatocytes), which involves beta(2) integrins and triggers the full activation of the neutrophil with a long-lasting adherence-dependent oxidant stress and degranulation. The oxidants diffuse into hepatocytes and trigger an intracellular oxidant stress, mitochondrial dysfunction and eventually cause oncotic necrotic cell death. Neutrophil-derived proteases facilitate extravasation and are involved in the regulation of inflammatory mediator production. Based on these mechanisms, it appears that strengthening of the intracellular defense mechanisms in hepatocytes may be the most promising therapeutic approach to selectively prevent neutrophil-mediated tissue damage without compromising their host-defense function.

Intrahepatic activation of caspases in human fulminant hepatic failure

BACKGROUND/AIMS

Apoptosis has been implicated in the pathogenesis of fulminant hepatic failure (FHF) potentially involving caspases. Thus far, apoptosis in FHF has mainly been studied in animal models while human data are sparse.

METHODS

Caspases-3, -8 and -9 activities and Fas expression were analyzed in correlation to TdT-mediated dUTP nick end labelling (TUNEL) positive apoptotic cells in livers of patients with FHF (n=26), chronic liver disease (CLD) (n=60) and normal controls (NC) (n=10).

RESULTS

Numbers of TUNEL-positive cells were higher in FHF than in CLD and NC (P<0.001) correlating to the intrahepatic activities of caspase-3. The highest caspase-3 activities were found in fulminant hepatitis B, significantly surpassing those in FHF of any other etiology. In fulminant hepatitis B, caspase-9 activity was also higher than in controls, while caspase-8 activation was not higher than in NC. Unlike caspase-3, caspases -8 and -9 activities were not correlated to the numbers of TUNEL positive cells. Fas expression was also the highest in FHF but did not differ between hepatitis B virus-FHF and other FHF.

CONCLUSIONS

Our data indicate differential activation of intrahepatic caspases in FHF depending on the underlying etiology. Massive activation of caspases in fulminant hepatitis B confirms a pivotal role of apoptotic pathways in the pathogenesis of human fulminant hepatitis B.

Critical role of plasminogen activator inhibitor-1 in cholestatic liver injury and fibrosis

Plasminogen activator inhibitor-1 (PAI-1) is an acute phase protein known to correlate with hepatic fibrosis. However, whether or not PAI-1 plays a causal role in this disease process had not been directly tested. Therefore, wild-type or PAI-1 knockout (PAI-1(-/-)) mice underwent bile duct ligation. Mice were sacrificed either 3 or 14 days after surgery for assessment of early (i.e., inflammation) and late (i.e., fibrosis) changes caused by bile duct ligation. Liver injury was determined by histopathology and plasma enzymes. Accumulation of extracellular matrix was evaluated by Sirius red staining and by measuring hydroxyproline content. Hepatic expression of PAI-1 was increased approximately 9-fold by bile duct ligation in wild-type mice. Furthermore, early liver injury and inflammation due to bile duct ligation was significantly blunted in PAI-1(-/-) mice in comparison with wild-type mice. Although PAI-1(-/-) mice were significantly protected against the accumulation of extracellular matrix caused by bile duct ligation, increases in expression of indices of stellate cell activation and collagen synthesis caused by bile duct ligation were not attenuated. Protection did, however, correlate with an elevation in hepatic activities of plasminogen activator and matrix metalloprotease activities. In contrast, the increase in tissue inhibitor of metalloproteases-1 protein, a major inhibitor of matrix metalloproteases, caused by bile duct ligation was not altered in PAI-1(-/-) mice compared with the wild-type strain. The increase in hepatic activity of urokinase-type plasminogen activator was also accompanied by more activation of the hepatocyte growth factor receptor c-Met. Taken together, these data suggest that PAI-1 plays a causal role in mediating fibrosis during cholestasis.

From alcohol toxicity to treatment

This article presents the proceedings of a symposium held at the meeting of the International Society for Biomedical Research on Alcoholism in Mannheim, Germany, in October 2004. This symposium was dedicated to Charles S. Lieber in recognition of his contribution in alcohol research over the last 50 years. It was divided into two parts, namely effects of alcohol on the gastrointestinal tract and effects of alcohol on the liver. Major emphasis was given to recent discoveries elucidating mechanisms of alcohol-associated carcinogenesis. M. Salaspuro (Finland) discussed the role of acetaldehyde in the saliva and in the large intestine with respect to its role in the pathogenesis of alcohol-associated cancer, and H. K. Seitz (Germany) presented new data identifying individuals homozygous for the ADH1C&1 allele as high on risk for alcohol-associated upper aerodigestive tract cancer. M. Savolainen (Finland) discussed the role phosphatidylethanol as a bioactive lipid that can mediate beneficial and harmful effects of alcohol drinking. In the second part of the symposium, alcoholic liver disease was discussed. P. Haber (Australia) presented new data on hepatic transcriptome in alcoholic liver disease with the identification of new genes possibly involved in alcohol-initiated fibrogenesis of the liver, and H. Moshage (The Netherlands) described survival mechanisms of the cholestatic hepatocytes with implications for therapy in cholestatic liver disease. The role of the hepatic microsomal ethanol oxidizing system in the metabolism of alcohol in alcoholic liver disease was summarized by R. Teschke (Germany). H. Ishii (Japan) discussed the current status and treatment of alcoholic hepatitis in Japan. Finally, in a state-of-the-art lecture, Charles S. Lieber (USA) discussed the development of the understanding of the pathophysiology of alcoholic liver disease in the last 50 years. He emphasized the role of pathophysiology as an important prerequisite for better treatment strategies.