Involvement of mitochondria in acetaminophen-induced apoptosis and hepatic injury: roles of cytochrome c, Bax, Bid, and caspases

Mechanisms of Acute Liver Failure

Acute liver failure (ALF) is characterized by the sudden onset of liver failure in a patient without evidence of chronic liver disease. This definition is important, as it differentiates patients with ALF from patients who suffer from liver failure owing to end-stage chronic liver disease [1].

An update on oxidative stress-mediated organ pathophysiology

Exposure to environmental pollutants and drugs can result in pathophysiological situations in the body. Research in this area is essential as the knowledge on cellular survival and death would help in designing effective therapeutic strategies that are needed for the maintenance of the normal physiological functions of the body. In this regard, naturally occurring bio-molecules can be considered as potential therapeutic targets as they are normally available in commonly consumed foodstuffs and are thought to have minimum side effects. This review article describes the detailed mechanisms of oxidative stress-mediated organ pathophysiology and the ultimate fate of the cells either to survive or to undergo necrotic or apoptotic death. The mechanisms underlying the beneficial role of a number of naturally occurring bioactive molecules in oxidative stress-mediated organ pathophysiology have also been included in the review. The review provides useful information about the recent progress in understanding the mechanism(s) of various types of organ pathophysiology, the complex cross-talk between these pathways, as well as their modulation in stressed conditions. Additionally, it suggests possible therapeutic applications of a number of naturally occurring bioactive molecules in conditions involving oxidative stress.

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.

Mechanism of the protective action of taurine in toxin and drug induced organ pathophysiology and diabetic complications: a review

Taurine (2-aminoethanesulfonic acid), a conditionally essential amino acid, is found in large concentrations in all mammalian tissues and is particularly abundant in aquatic foods. Taurine exhibits membrane stabilizing, osmoregulatory and cytoprotective effects, antioxidative properties, regulates intracellular Ca(2+) concentration, modulates ion movement and neurotransmitters, reduce the levels of pro-inflammatory cytokines in various organs and controls blood pressure. Recently, emerging evidence from the literature shows the effectiveness of taurine as a protective agent against several environmental toxins and drug-induced multiple organ injuries as the outcome of hepatotoxicity, nephrotoxicity, neurotoxicity, testicular toxicity and cardiotoxicity in several animal models. Besides, taurine is also effective in combating diabetes and its associated complications, including cardiomyopathy, nephropathy, neuropathy, retinopathy and atherosclerosis. These beneficial effects appear to be due to the multiple actions of taurine on cellular functions. This review summarizes the mechanism of the prophylactic role of taurine against several environmental toxins and drug-induced organ pathophysiology and diabetes.

ARC is a novel therapeutic approach against acetaminophen-induced hepatocellular necrosis

BACKGROUND & AIMS

Acetaminophen (AAP) overdose is the most frequent cause of drug-induced liver failure. c-Jun N-terminal kinase (JNK) is thought to play a central role in AAP-induced hepatocellular necrosis. The apoptosis repressor with caspase recruitment domain (ARC) is a death repressor that inhibits death receptor and mitochondrial apoptotic signaling. Here, we investigated ARC's therapeutic effect and molecular mechanisms on AAP-induced hepatocellular necrosis.

METHODS

We tested the in vivo and in vitro effects of ARC fused with the transduction domain of HIV-1 (TAT-ARC) on murine AAP hepatotoxicity.

RESULTS

Treatment with TAT-ARC protein completely abrogated otherwise lethal liver failure induced by AAP overdose in C57BL/6 mice. AAP triggered caspase-independent necrosis, as evidenced by liver histology, elevated serum transaminases, and secreted HMGB1 that was inhibited by ARC. ARC-mediated hepatoprotection was not caused by an alteration of AAP metabolism, but resulted in reduced oxidative stress. AAP overdose led to induction of RIP-dependent signaling with subsequent JNK activation. Ectopic ARC inhibited JNK activation by specific interactions between ARC and JNK1 and JNK2. Importantly, survival of mice was even preserved when ARC therapy was initiated in a delayed manner after AAP administration.

CONCLUSIONS

This work identifies for the first time ARC-JNK-binding with subsequent inhibition of JNK signaling as a specific mechanism of ARC to interfere with AAP-dependent necrosis. Our data suggests that AAP-mediated induction of RIP signaling serves as a critical switch for hepatocellular necrosis. The efficacy of TAT-ARC protein transduction in murine AAP hepatotoxicity suggests its therapeutic potential for reversing AAP intoxication also in humans.

Models of drug-induced liver injury for evaluation of phytotherapeutics and other natural products

Extracts from medicinal plants, many of which have been used for centuries, are increasingly tested in models of hepatotoxicity. One of the most popular models to evaluate the hepatoprotective potential of natural products is acetaminophen (APAP)-induced liver injury, although other hepatotoxicity models such as carbon tetrachloride, thioacetamide, ethanol and endotoxin are occasionally used. APAP overdose is a clinically relevant model of drug-induced liver injury. Critical mechanisms and signaling pathways, which trigger necrotic cell death and sterile inflammation, are discussed. Although there is increasing understanding of the pathophysiology of APAP-induced liver injury, the mechanism is complex and prone to misinterpretation, especially when unknown chemicals such as plant extracts are tested. This review discusses the fundamental aspects that need to be considered when using this model, such as selection of the animal species or in vitro system, timing and dose-responses of signaling events, metabolic activation and protein adduct formation, the role of lipid peroxidation and apoptotic versus necrotic cell death, and the impact of the ensuing sterile inflammatory response. The goal is to enable researchers to select the appropriate model and experimental conditions for testing of natural products that will yield clinically relevant results and allow valid interpretations of the pharmacological mechanisms.

TRAIL enhances paracetamol-induced liver sinusoidal endothelial cell death in a Bim- and Bid-dependent manner

Paracetamol (acetaminophen, APAP) is a universally used analgesic and antipyretic agent. Considered safe at therapeutic doses, overdoses cause acute liver damage characterized by centrilobular hepatic necrosis. One of the major clinical problems of paracetamol-induced liver disease is the development of hemorrhagic alterations. Although hepatocytes represent the main target of the cytotoxic effect of paracetamol overdose, perturbations within the endothelium involving morphological changes of liver sinusoidal endothelial cells (LSECs) have also been described in paracetamol-induced liver disease. Recently, we have shown that paracetamol-induced liver damage is synergistically enhanced by the TRAIL signaling pathway. As LSECs are constantly exposed to activated immune cells expressing death ligands, including TRAIL, we investigated the effect of TRAIL on paracetamol-induced LSEC death. We here demonstrate for the first time that TRAIL strongly enhances paracetamol-mediated LSEC death with typical features of apoptosis. Inhibition of caspases using specific inhibitors resulted in a strong reduction of cell death. TRAIL appears to enhance paracetamol-induced LSEC death via the activation of the pro-apoptotic BH3-only proteins Bid and Bim, which initiate the mitochondrial apoptotic pathway. Taken together this study shows that the liver endothelial layer, mainly LSECs, represent a direct target of the cytotoxic effect of paracetamol and that activation of TRAIL receptor synergistically enhances paracetamol-induced LSEC death via the mitochondrial apoptotic pathway. TRAIL-mediated acceleration of paracetamol-induced cell death may thus contribute to the pathogenesis of paracetamol-induced liver damage.

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.

Development of an accurate index for predicting outcomes of patients with acute liver failure

BACKGROUND & AIMS

Patients with acute liver failure (ALF) have high mortality and frequently require liver transplantation (LT); few reliable prognostic markers are available. Levels of M30, a cleavage product of cytokeratin-18 caspase, are significantly increased in serum samples from patients with ALF who die or undergo LT. We developed a prognostic index for ALF based on level of M30 and commonly measured clinical variables (called the Acute Liver Failure Study Group [ALFSG] index) and compared its accuracy with that of the King's College criteria (KCC) and Model for End Stage Liver Disease (MELD). We also validated our model in an independent group of patients with ALF.

METHODS

Serum levels of M30 and M65 antigen (the total cytokeratin-18 fragment, a marker of apoptosis and necrosis) were measured on 3 of the first 4 days following admission of 250 patients with ALF. Logistic regression was used to determine whether the following factors, measured on day 1, were associated with LT or death: age, etiology; coma grade; international normalized ratio (INR); serum pH; body mass index; levels of creatinine, bilirubin, phosphorus, arterial ammonia, and lactate; and log(10) M30 and log(10) M65. The area under the receiver operating characteristic (AUROC) was calculated for the ALFSG and other indices.

RESULTS

Coma grade, INR, levels of bilirubin and phosphorus, and log(10) M30 value at study entry most accurately identified patients who would require LT or die. The ALFSG index identified these patients with 85.6% sensitivity and 64.7% specificity. Based on comparison of AUROC values, the ALFSG Index (AUROC, 0.822) better identified patients most likely to require LT or die than the KCC (AUROC, 0.654) or MELD (AUROC, 0.704) (P = .0002 and P = .0010, respectively). We validated these findings in a separate group of 250 patients with ALF.

CONCLUSIONS

The ALFSG index, a combination of clinical markers and measurements of the apoptosis biomarker M30, better predicts outcomes of patients with ALF than the KCC or MELD.

Dioscin, a natural steroid saponin, shows remarkable protective effect against acetaminophen-induced liver damage in vitro and in vivo

The aim of the study was to investigate the protective effect of dioscin against APAP-induced hepatotoxicity. In the in vitro tests, HepG2 cells were given APAP pretreatment with or without dioscin. In the in vivo experiments, mice were orally administrated dioscin for five days and then given APAP. Some biochemical and morphology parameters were assayed and the possible mechanism was investigated. Dioscin improved AST release, mitochondrial dysfunction, apoptosis and necrosis of HepG2 cells induced by APAP. Following administration of dioscin, APAP-induced hepatotoxicity in mice was significantly attenuated. Furthermore, the liver cell apoptosis and necrosis, and hepatic mitochondrial edema were also prevented. Fifteen differentially expressed proteins were found by using proteomics, and six of them, Suox, Krt18, Rgn, Prdx1, MDH and PNP were validated. These proteins may be involved in the hepatoprotective effect of dioscin and might cooperate with the levels of Ca(2+) in mitochondria, decreased expression of ATP2A2, and decreased mitochondrial cardiolipin. In addition, dioscin inhibited APAP-induced activation and expression of CYP2E1, up-regulated the expression of Bcl-2 and Bid, and inhibited the expression of Bax, Bak and p53. Dioscin showed a remarkable protective effect against APAP-induced hepatotoxicity by adjusting mitochondrial function. These results indicated that dioscin has the capability on the treatment of liver injury.

Novel protective mechanisms for S-adenosyl-L-methionine against acetaminophen hepatotoxicity: improvement of key antioxidant enzymatic function

Acetaminophen (APAP) overdose leads to severe hepatotoxicity, increased oxidative stress and mitochondrial dysfunction. S-adenosyl-L-methionine (SAMe) protects against APAP toxicity at a mmol/kg equivalent dose to N-acetylcysteine (NAC). SAMe acts as a principle biological methyl donor and participates in polyamine synthesis which increase cell growth and has a role in mitochondrial protection. The purpose of the current study tested the hypothesis that SAMe protects against APAP toxicity by maintaining critical antioxidant enzymes and markers of oxidative stress. Male C57Bl/6 mice were treated with vehicle (Veh; water 15 ml/kg, ip), SAMe (1.25 mmol/kg, ip), APAP (250 mg/kg, ip), and SAMe+APAP (SAMe given 1 h following APAP). Liver was collected 2 and 4 h following APAP administration; mitochondrial swelling as well as hepatic catalase, glutathione peroxidase (GPx), glutathione reductase, and both Mn- and Cu/Zn-superoxide dismutase (SOD) enzyme activity were evaluated. Mitochondrial protein carbonyl, 3-nitrotyrosine cytochrome c leakage were analyzed by Western blot. SAMe significantly increased SOD, GPx, and glutathione reductase activity at 4 h following APAP overdose. SAMe greatly reduced markers of oxidative stress and cytochrome C leakage following APAP overdose. Our studies also demonstrate that a 1.25 mmol/kg dose of SAMe does not inhibit CYP 2E1 enzyme activity. The current study identifies a plausible mechanism for the decreased oxidative stress observed when SAMe is given following APAP.

Mechanisms of cell death in acute liver failure

Acute liver failure (ALF) can be the consequence of various etiologies, that might vary between different geographic regions. Most frequent are intoxications with acetaminophen, viral hepatitis, or liver damage of unknown origin. ALF occurs when the extent of hepatocyte death exceeds the regenerative capacity of the liver. The mode of liver cell death that is predominantly induced in ALF, i.e., apoptosis or necrosis, is still controversial and presumably determined by the etiology, duration, and magnitude of liver injury. Severe liver damage involves oxidative stress and depletion of ATP resulting in necrosis. In contrast, maintenance of ATP stores is required for the execution of apoptosis. Recent data suggest that necrosis resulting from severe liver damage is associated with poor outcome of ALF patients. Discrimination between apoptosis and necrosis might be therefore useful for the identification of ALF patients requiring liver transplantation. Identification of the molecular cell death mechanisms remains an important issue not only for early prediction of ALF outcome, but also for therapeutic interventions. In view of the pleiotropic functions of critical mediators of cell death and tissue regeneration, a particular challenge will be to reduce hepatocellular death without inhibiting the regenerative capacity of the liver. Here, we review the molecular mechanisms of hepatocyte injury and the pathways leading to apoptosis and necrosis, which might represent potential diagnostic and therapeutic targets in ALF.

Oxidant stress, mitochondria, and cell death mechanisms in drug-induced liver injury: lessons learned from acetaminophen hepatotoxicity

Hepatotoxicity is a serious problem during drug development and for the use of many established drugs. For example, acetaminophen overdose is currently the most frequent cause of acute liver failure in the United States and Great Britain. Evaluation of the mechanisms of drug-induced liver injury indicates that mitochondria are critical targets for drug toxicity, either directly or indirectly through the formation of reactive metabolites. The consequence of these modifications is generally a mitochondrial oxidant stress and peroxynitrite formation, which leads to structural alterations of proteins and mitochondrial DNA and, eventually, to the opening of mitochondrial membrane permeability transition (MPT) pores. MPT pore formation results in a collapse of mitochondrial membrane potential and cessation of adenosine triphosphate synthesis. In addition, the release of intermembrane proteins, such as apoptosis-inducing factor and endonuclease G, and their translocation to the nucleus, leads to nuclear DNA fragmentation. Together, these events trigger necrotic cell death. Alternatively, the release of cytochrome c and other proapoptotic factors from mitochondria can promote caspase activation and apoptotic cell death. Drug toxicity can also induce an inflammatory response with the formation of reactive oxygen species by Kupffer cells and neutrophils. If not properly detoxified, these extracellularly generated oxidants can diffuse into hepatocytes and trigger mitochondrial dysfunction and oxidant stress, which then induces MPT and necrotic cell death. This review addresses the formation of oxidants and the defense mechanisms available for cells and applies this knowledge to better understand mechanisms of drug hepatotoxicity, especially acetaminophen-induced liver injury.

Mouse strain-dependent caspase activation during acetaminophen hepatotoxicity does not result in apoptosis or modulation of inflammation

The mechanisms of acetaminophen (APAP)-mediated hepatic oncotic necrosis have been extensively characterized. However, it was recently demonstrated that fed CD-1 mice have a transient caspase activation which initiates apoptosis. To evaluate these findings in more detail, outbred (Swiss Webster, SW) and inbred (C57BL/6) mice were treated with APAP with or without pan-caspase inhibitor and compared to the apoptosis model of galactosamine (GalN)/endotoxin (ET). Fasted or fed APAP-treated C57BL/6 mice showed no evidence of caspase-3 processing or activity. Interestingly, a minor, temporary increase in caspase-3 processing and activity (150% above baseline) was observed after APAP treatment only in fed SW mice. The degree of caspase-3 activation in SW mice after APAP was minor compared to that observed in GalN/ET-treated mice (1600% above baseline). The pancaspase inhibitor attenuated caspase activation and resulted in increased APAP-induced injury (plasma ALT, necrosis scoring). The caspase inhibitor did not affect apoptosis because regardless of treatment only <0.5% of hepatocytes showed consistent apoptotic morphology after APAP. In contrast, >20% apoptotic cells were observed in GalN/ET-treated mice. Presence of the caspase inhibitor altered hepatic glutathione levels in SW mice, which could explain the exacerbation of injury. Additionally, the infiltration of hepatic neutrophils was not altered by the fed state of either mouse strain.

CONCLUSION

Minor caspase-3 activation without apoptotic cell death can be observed only in fed mice of some outbred strains. These findings suggest that although the severity of APAP-induced liver injury varies between fed and fasted animals, the mechanism of cell death does not fundamentally change.

Acetaminophen overdose-induced liver injury in mice is mediated by peroxynitrite independently of the cyclophilin D-regulated permeability transition

Acetaminophen (APAP) is safe at therapeutic dosage but can cause severe hepatotoxicity if used at overdose. The mechanisms of injury are not yet fully understood, but previous reports had suggested that the mitochondrial permeability transition (mPT) may be involved in triggering hepatocellular necrosis. We aimed at inhibiting mitochondrial cyclophilin D (CypD), a key regulator of the mPT, as a potential therapeutic target in APAP hepatotoxicity. Wildtype mice treated with a high dose of APAP (600 mg/kg, intraperitoneal) developed typical centrilobular necrosis, which could not, however, be prevented by cotreatment with the selective CypD inhibitor, Debio 025 (alisporivir, DEB025, a nonimmunosuppressive cyclosporin A analog). Similarly, genetic ablation of mitochondrial CypD in Ppif-null mice did not afford protection from APAP hepatotoxicity. To determine whether APAP-induced peroxynitrite stress might directly activate mitochondrial permeabilization, independently of the CypD-regulated mPT, we coadministered the peroxynitrite decomposition catalyst Fe-TMPyP (10 mg/kg, intraperitoneal, 90 minutes prior to APAP) to CypD-deficient mice. Liver injury was greatly attenuated by Fe-TMPyP pretreatment, and mitochondrial 3-nitrotyrosine adduct levels (peroxynitrite marker) were decreased. Acetaminophen treatment increased both the cytosolic and mitochondria-associated P-JNK levels, but the c-jun-N-terminal kinase (JNK) signaling inhibitor SP600125 was hepatoprotective in wildtype mice only, indicating that the JNK pathway may not be critically involved in the absence of CypD.

CONCLUSION

These data support the concept that an overdose of APAP results in liver injury that is refractory to pharmacological inhibition or genetic depletion of CypD and that peroxynitrite-mediated cell injury predominates in the absence of CypD.

Distinguishing drug-induced minor morphological changes from major cellular damage via label-free impedimetric toxicity screening

We present a novel perfusion-based microfluidic platform for label-free drug toxicity screening which can single out non-lethal morphological changes from cellular death using electrical impedance spectroscopy. Minor cellular changes such as cell-cell contacts and major cell injury were identified via impedance phase angle analysis and follow-up of impedance magnitude at different frequencies. Having exposed HepG2/C3A cells to acetaminophen (AP), we showed that continuous drug perfusion caused a time and concentration-dependent impedance decrease. Moreover, perfusion of repeated doses revealed altered dielectric properties of the cell culture after recovery from AP exposure. This study highlights the possibility to sense cellular changes long before cellular death takes place, pointing out the remarkable sensitivity advantage of this technique over standard endpoint viability tests and its interest for toxicology.

Role of TRAIL and the pro-apoptotic Bcl-2 homolog Bim in acetaminophen-induced liver damage

Acetaminophen (N-acetyl-para-aminophenol (APAP), paracetamol) is a commonly used analgesic and antipyretic agent. Although considered safe at therapeutic doses, accidental or intentional overdose causes acute liver failure characterized by centrilobular hepatic necrosis with high morbidity and mortality. Although many molecular aspects of APAP-induced cell death have been described, no conclusive mechanism has been proposed. We recently identified TNF-related apoptosis-inducing ligand (TRAIL) and c-Jun kinase (JNK)-dependent activation of the pro-apoptotic Bcl-2 homolog Bim as an important apoptosis amplification pathway in hepatocytes. In this study, we, thus, investigated the role of TRAIL, c-JNK and Bim in APAP-induced liver damage. Our results demonstrate that TRAIL strongly synergizes with APAP in inducing cell death in hepatocyte-like cells lines and primary hepatocyte. Furthermore, we found that APAP strongly induces the expression of Bim in a c-JNK-dependent manner. Consequently, TRAIL- or Bim-deficient mice were substantially protected from APAP-induced liver damage. This study identifies the TRAIL-JNK-Bim axis as a novel target in the treatment of APAP-induced liver damage and substantiates its general role in hepatocyte death.

Current issues with acetaminophen hepatotoxicity--a clinically relevant model to test the efficacy of natural products

There is a significant need to evaluate the therapeutic potential of natural products and other compounds purported to be hepatoprotective. Acetaminophen-induced liver injury, especially in mice, is an attractive and widely used model for this purpose because it is both clinically relevant and experimentally convenient. However, the pathophysiology of liver injury after acetaminophen overdose is complex. This review describes the multiple steps and signaling pathways involved in acetaminophen-mediated cell death. The toxicity is initiated by the formation of a reactive metabolite, which depletes glutathione and binds to cellular proteins, especially in mitochondria. The resulting mitochondrial oxidant stress and peroxynitrite formation, in part through amplification by c-jun-N-terminal kinase activation, leads to mitochondrial DNA damage and opening of the mitochondrial permeability transition pore. Endonucleases from the mitochondrial intermembrane space and lysosomes are responsible for nuclear DNA fragmentation. Despite the oxidant stress, lipid peroxidation is not a relevant mechanism of injury. The mitochondrial dysfunction and nuclear DNA damage ultimately cause oncotic necrotic cell death with release of damage-associated molecular patterns that trigger a sterile inflammatory response. Current evidence supports the hypothesis that innate immune cells do not contribute to injury but are involved in cell debris removal and regeneration. This review discusses the latest mechanistic aspects of acetaminophen hepatotoxicity and demonstrates ways to assess the mechanisms of drug action and design experiments needed to avoid pitfalls and incorrect conclusions. This review should assist investigators in the optimal use of this model to test the efficacy of natural compounds and obtain reliable mechanistic information.

Cyclophilin D deficiency protects against acetaminophen-induced oxidant stress and liver injury

Acetaminophen (APAP) hepatotoxicity is the main cause of acute liver failure in humans. Although mitochondrial oxidant stress and induction of the mitochondrial permeability transition (MPT) have been implicated in APAP-induced hepatotoxicity, the link between these events is unclear. To investigate this, this study evaluated APAP hepatotoxicity in mice deficient of cyclophilin D, a protein component of the MPT. Treatment of wild type mice with APAP resulted in focal centrilobular necrosis, nuclear DNA fragmentation and formation of reactive oxygen (elevated glutathione disulphide levels) and peroxynitrite (nitrotyrosine immunostaining) in the liver. CypD-deficient (Ppif(-/-)) mice were completely protected against APAP-induced liver injury and DNA fragmentation. Oxidant stress and peroxynitrite formation were blunted but not eliminated in CypD-deficient mice. Thus, mitochondrial oxidative stress and induction of the MPT are critical events in APAP hepatotoxicity in vivo and at least part of the APAP-induced oxidant stress and peroxynitrite formation occurs downstream of the MPT.

The impact of partial manganese superoxide dismutase (SOD2)-deficiency on mitochondrial oxidant stress, DNA fragmentation and liver injury during acetaminophen hepatotoxicity

Acetaminophen (APAP) hepatotoxicity is the most frequent cause of acute liver failure in many countries. The mechanism of cell death is initiated by formation of a reactive metabolite that binds to mitochondrial proteins and promotes mitochondrial dysfunction and oxidant stress. Manganese superoxide dismutase (SOD2) is a critical defense enzyme located in the mitochondrial matrix. The objective of this investigation was to evaluate the functional consequences of partial SOD2-deficiency (SOD2+/-) on intracellular signaling mechanisms of necrotic cell death after APAP overdose. Treatment of C57Bl/6J wild type animals with 200mg/kg APAP resulted in liver injury as indicated by elevated plasma alanine aminotransferase activities (2870±180U/L) and centrilobular necrosis at 6h. In addition, increased tissue glutathione disulfide (GSSG) levels and GSSG-to-GSH ratios, delayed mitochondrial GSH recovery, and increased mitochondrial protein carbonyls and nitrotyrosine protein adducts indicated mitochondrial oxidant stress. In addition, nuclear DNA fragmentation (TUNEL assay) correlated with translocation of Bax to the mitochondria and release of apoptosis-inducing factor (AIF). Furthermore, activation of c-jun-N-terminal kinase (JNK) was documented by the mitochondrial translocation of phospho-JNK. SOD2+/- mice showed 4-fold higher ALT activities and necrosis, an enhancement of all parameters of the mitochondrial oxidant stress, more AIF release and more extensive DNA fragmentation and more prolonged JNK activation.

CONCLUSIONS

the impaired defense against mitochondrial superoxide formation in SOD2+/- mice prolongs JNK activation after APAP overdose and consequently further enhances the mitochondrial oxidant stress leading to exaggerated mitochondrial dysfunction, release of intermembrane proteins with nuclear DNA fragmentation and more necrosis.

Modulation of Bax/Bcl-2 and caspases by probiotics during acetaminophen induced apoptosis in primary hepatocytes

Oxidative stress is an important factor in drug induced hepatotoxicity and antioxidants from natural sources have potential to ameliorate it. The present study was aimed to investigate cyto-protective potential of probiotic Enterococcus lactis IITRHR1 (El(SN)) and Lactobacillus acidophilus MTCC447 (La(SN)) lysate against acetaminophen (APAP) induced hepatotoxicity. Cultured rat hepatocytes pretreated with El(SN)/La(SN) showed higher cell viability under APAP stress. Pre-treatment with El(SN,) restored glutathione level and reduced ROS generation significantly which are major biomarkers of oxidative stress. It also reduced NO level, MDA formation and enhanced SOD activity. Pre-treatment with probiotic lysates significantly inhibited the translocation of pro-apoptotic protein (Bax), enhanced anti-apoptotic (Bcl-2) protein levels and prevented release of cyt c to cytosol; suggesting involvement of mitochondrial proteins in protection against APAP induced oxidative cellular damage. Loss in mitochondrial membrane potential due to APAP treatment was prevented in the presence of probiotic lysates. Protective action of El(SN)/La(SN) pretreatment was further supported by prevention of procaspase-3 activation, DNA fragmentation and chromatin condensation, in turn inhibiting APAP induced apoptotic cell death. The results indicate that probiotic preparations modulate crucial end points of oxidative stress induced apoptosis and may be used for management of drug induced liver injury.

Influence of small doses of various drug vehicles on acetaminophen-induced liver injury

The biological effects of drug vehicles are often overlooked, often leading to artifacts in acetaminophen-induced liver injury assessment. Therefore, we decided to investigate the effect of dimethylsulfoxide, dimethylformamide, propylene glycol, ethanol, and Tween 20 on acetaminophen-induced liver injury. C57BL/6 male mice received a particular drug vehicle (0.6 or 0.2 mL/kg, i.p.) 30 min before acetaminophen administration (300 mg/kg, i.p.). Control mice received vehicle alone. Liver injury was assessed by measuring the concentration of alanine aminotransferase in plasma and observing histopathological changes. The level of reduced glutathione (GSH) was assessed by measuring total nonprotein hepatic sulfhydrils. Dimethylsulfoxide and dimethylformamide (at both doses) almost completely abolished acetaminophen toxicity. The higher dose of propylene glycol (0.6 mL/kg) was markedly protective, but the lower dose (0.2 mL/kg) was only slightly protective. These solvents also reduced acetaminophen-induced GSH depletion. Dimethylformamide was protective when given 2 h before or 1 h after acetaminophen administration, but was ineffective if given 2.5 h after acetaminophen. Ethanol at the higher dose (0.6 mL/kg) was partially protective, whereas ethanol at the lower dose (0.2 mL/kg) as well as Tween 20 at any dose had no influence. None of the vehicles (0.6 mL/kg) was hepatotoxic per se, and none of them was protective in a model of liver injury caused by d-galactosamine and lipopolysaccharide.

Diet restriction inhibits apoptosis and HMGB1 oxidation and promotes inflammatory cell recruitment during acetaminophen hepatotoxicity

Acetaminophen (APAP) overdose is a major cause of acute liver failure and serves as a paradigm to elucidate mechanisms, predisposing factors and therapeutic interventions. The roles of apoptosis and inflammation during APAP hepatotoxicity remain controversial. We investigated whether fasting of mice for 24 h can inhibit APAP-induced caspase activation and apoptosis through the depletion of basal ATP. We also investigated in fasted mice the critical role played by inhibition of caspase-dependent cysteine 106 oxidation within high mobility group box-1 protein (HMGB1) released by ATP depletion in dying cells as a mechanism of immune activation. In fed mice treated with APAP, necrosis was the dominant form of hepatocyte death. However, apoptosis was also observed, indicated by K18 cleavage, DNA laddering and procaspase-3 processing. In fasted mice treated with APAP, only necrosis was observed. Inflammatory cell recruitment as a consequence of hepatocyte death was observed only in fasted mice treated with APAP or fed mice cotreated with a caspase inhibitor. Hepatic inflammation was also associated with loss in detection of serum oxidized-HMGB1. A significant role of HMGB1 in the induction of inflammation was confirmed with an HMGB1-neutralizing antibody. The differential response between fasted and fed mice was a consequence of a significant reduction in basal hepatic ATP, which prevented caspase processing, rather than glutathione depletion or altered APAP metabolism. Thus, the inhibition of caspase-driven apoptosis and HMGB1 oxidation by ATP depletion from fasting promotes an inflammatory response during drug-induced hepatotoxicity/liver pathology.

Acetaminophen induced acute liver failure via oxidative stress and JNK activation: protective role of taurine by the suppression of cytochrome P450 2E1

The present study was carried out to investigate whether taurine plays any beneficial role in acetaminophen (APAP)-induced acute hepatotoxicity. APAP exposure increased the plasma levels of ALT, ALP, LDH, TNF-alpha and NO production. Moreover, APAP treatment reduced the glutathione level and antioxidant enzyme activities, increased lipid peroxidation and caused hepatic DNA fragmentation which ultimately leads to cellular necrosis. Also, incubation of hepatocytes with APAP reduced cell viability, enhanced ROS generation and increased CYP2E1 activity. APAP overdose caused injury in the hepatic tissue and hepatocytes via the upregulation of CYP2E1 and JNK. Taurine treatment was effective in counteracting APAP-induced hepatic damages, oxidative stress and cellular necrosis. Results indicate that APAP overdose caused hepatic injury due to its metabolism to hepatotoxic NAPQI (N-acetyl-p-benzoquinone imine), usually catalysed by CYP2E1, and via the direct activation of JNK-dependent cell death pathway. Taurine possesses prophylactic as well as therapeutic potentials against APAP-induced hepatic injury.

Mechanisms of acetaminophen-induced liver necrosis

Although considered safe at therapeutic doses, at higher doses, acetaminophen produces a centrilobular hepatic necrosis that can be fatal. Acetaminophen poisoning accounts for approximately one-half of all cases of acute liver failure in the United States and Great Britain today. The mechanism occurs by a complex sequence of events. These events include: (1) CYP metabolism to a reactive metabolite which depletes glutathione and covalently binds to proteins; (2) loss of glutathione with an increased formation of reactive oxygen and nitrogen species in hepatocytes undergoing necrotic changes; (3) increased oxidative stress, associated with alterations in calcium homeostasis and initiation of signal transduction responses, causing mitochondrial permeability transition; (4) mitochondrial permeability transition occurring with additional oxidative stress, loss of mitochondrial membrane potential, and loss of the ability of the mitochondria to synthesize ATP; and (5) loss of ATP which leads to necrosis. Associated with these essential events there appear to be a number of inflammatory mediators such as certain cytokines and chemokines that can modify the toxicity. Some have been shown to alter oxidative stress, but the relationship of these modulators to other critical mechanistic events has not been well delineated. In addition, existing data support the involvement of cytokines, chemokines, and growth factors in the initiation of regenerative processes leading to the reestablishment of hepatic structure and function.

Gap junction dysfunction reduces acetaminophen hepatotoxicity with impact on apoptotic signaling and connexin 43 protein induction in rat

Acetaminophen (APAP) is a widely used antipyretic and analgesic agent. However, overdosing and sometimes even a recommended dose can lead to serious and conceivably fatal liver toxicity. Therefore, it is important to clarify understand mechanisms of hepatotoxicity induced by APAP. Gap junctions, formed by connexin, have important roles in maintenance of tissue homeostasis and control of cell growth and differentiation. In the liver, Cx32 is a major gap junction protein whose expression is known to gradually decrease with chronic liver disease progression. In the present study, acute hepatotoxic effects of APAP were found to be reduced in Cx32 dominant negative transgenic rats lacking normal gap junctional intercellular communication in the liver. In littermate wild-type rats, the injured centrilobular hepatocytes were positive for TUNEL staining and featured elevated expre ssion of cleaved caspase-3 and Cx43, which is not expressed in normal hepatocytes. These results suggest that APAP hepatotoxicity involves apoptosis, and induction of Cx43 expression may play an important role in the apoptotic signaling. Moreover, gap junctional functions of Cx32 can play important roles in removing damaged hepatocytes by apoptosis for liver tissue homeostasis.

Pathogenic role of natural killer T and natural killer cells in acetaminophen-induced liver injury in mice is dependent on the presence of dimethyl sulfoxide

Dimethyl sulfoxide (DMSO) is commonly used in biological studies to dissolve drugs and enzyme inhibitors with low solubility. Although DMSO is generally thought of as being relatively inert, it can induce biological effects that are often overlooked. An example that highlights this potential problem is found in a recent report demonstrating a pathogenic role for natural killer T (NKT) and natural killer (NK) cells in acetaminophen-induced liver injury (AILI) in C57Bl/6 mice in which DMSO was used to facilitate acetaminophen (APAP) dissolution. We report that NKT and NK cells do not play a pathologic role in AILI in C57Bl/6 mice in the absence of DMSO. Although AILI was significantly attenuated in mice depleted of NKT and NK cells prior to APAP treatment in the presence of DMSO, no such effect was observed when APAP was dissolved in saline. Because of this unexpected finding, the effects of DMSO on hepatic NKT and NK cells were subsequently investigated. When given alone, DMSO activated hepatic NKT and NK cells in vivo as evidenced by increased NKT cell numbers and higher intracellular levels of the cytotoxic effector molecules interferon-gamma (IFN-gamma) and granzyme B in both cell types. Similarly, when used as a solvent for APAP, DMSO again increased NKT cell numbers and induced IFN-gamma and granzyme B expression in both cell types.

CONCLUSION

These data demonstrate a previously unappreciated effect of DMSO on hepatic NKT and NK cells, suggesting that DMSO should be used cautiously in experiments involving these cells.

Opposing roles for caspase and calpain death proteases in L-glutamate-induced oxidative neurotoxicity

Oxidative glutamate toxicity in HT22 murine hippocampal cells is a model for neuronal death by oxidative stress. We have investigated the role of proteases in HT22 cell oxidative glutamate toxicity. L-glutamate-induced toxicity was characterized by cell and nuclear shrinkage and chromatin condensation, yet occurred in the absence of either DNA fragmentation or mitochondrial cytochrome c release. Pretreatment with the selective caspase inhibitors either benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (pan-caspase), N-acetyl-Leu-Glu-His-Asp-aldehyde (caspase 9) or N-acetyl-Ile-Glu-Thr-Asp-aldehyde (caspase 8), significantly increased L-glutamate-induced cell death with a corresponding increase in observed nuclear shrinkage and chromatin condensation. This enhancement of glutamate toxicity correlated with an increase in L-glutamate-dependent production of reactive oxygen species (ROS) as a result of caspase inhibition. Pretreating the cells with N-acetyl-L-cysteine prevented ROS production, cell shrinkage and cell death from L-glutamate as well as that associated with the presence of the pan-caspase inhibitor. In contrast, the caspase-3/-7 inhibitor N-acetyl-Asp-Glu-Val-Asp aldehyde was without significant effect. However, pretreating the cells with the calpain inhibitor N-acetyl-Leu-Leu-Nle-CHO, but not the cathepsin B inhibitor CA-074, prevented cell death. The cytotoxic role of calpains was confirmed further by: 1) cytotoxic dependency on intracellular Ca(2+) increase, 2) increased cleavage of the calpain substrate Suc-Leu-Leu-Val-Tyr-AMC and 3) immunoblot detection of the calpain-selective 145 kDa alpha-fodrin cleavage fragment. We conclude that oxidative L-glutamate toxicity in HT22 cells is mediated via calpain activation, whereas inhibition of caspases-8 and -9 may exacerbate L-glutamate-induced oxidative neuronal damage through increased oxidative stress.

Stem cell factor and c-kit are involved in hepatic recovery after acetaminophen-induced liver injury in mice

Stem cell factor (SCF) and its receptor c-kit are important in hematopoiesis and cellular proliferation. c-kit has also been identified as a cell surface marker for progenitor cells. We have previously shown that there is a large reservoir of hepatic SCF, and this molecule plays a significant role in liver regeneration after 70% hepatectomy. In the current study, we further examined the expression of SCF and c-kit in acetaminophen (APAP)-induced liver injury in C57BL/6J mice or SCF-deficient sl-sld mice and their appropriate wild-type controls. Following APAP-induced liver injury, c-kit mRNA expression increased, with peak levels detected 48 h postinjury. Hepatic SCF mRNA levels after APAP injury were also increased, with peak levels seen 16 h post-APAP. The mortality rate in SCF-deficient mice treated with APAP was significantly higher than that of wild-type mice; furthermore, administration of exogenous SCF significantly reduced the mortality of APAP-treated wild-type mice. Bromodeoxyuridine incorporation experiments showed that SCF significantly increased hepatocyte proliferation at 48 and 72 h in APAP-treated mice. SCF inhibited APAP-induced hepatocyte apoptosis and increased Bcl-2 and Bcl-xL expression, suggesting that this decrease in hepatocyte apoptosis is mediated through Bcl-2 and Bcl-xL. In summary, SCF and c-kit expression was increased after APAP-induced liver injury. Administration of exogenous SCF reduces mortality in APAP-treated mice, increases hepatocyte proliferation, and prevents hepatocyte apoptosis induced by APAP, suggesting that these molecules are important in the liver's recovery from these injuries.

Stem cell factor and c-kit are involved in hepatic recovery after acetaminophen-induced liver injury in mice

Stem cell factor (SCF) and its receptor c-kit are important in hematopoiesis and cellular proliferation. c-kit has also been identified as a cell surface marker for progenitor cells. We have previously shown that there is a large reservoir of hepatic SCF, and this molecule plays a significant role in liver regeneration after 70% hepatectomy. In the current study, we further examined the expression of SCF and c-kit in acetaminophen (APAP)-induced liver injury in C57BL/6J mice or SCF-deficient sl-sld mice and their appropriate wild-type controls. Following APAP-induced liver injury, c-kit mRNA expression increased, with peak levels detected 48 h postinjury. Hepatic SCF mRNA levels after APAP injury were also increased, with peak levels seen 16 h post-APAP. The mortality rate in SCF-deficient mice treated with APAP was significantly higher than that of wild-type mice; furthermore, administration of exogenous SCF significantly reduced the mortality of APAP-treated wild-type mice. Bromodeoxyuridine incorporation experiments showed that SCF significantly increased hepatocyte proliferation at 48 and 72 h in APAP-treated mice. SCF inhibited APAP-induced hepatocyte apoptosis and increased Bcl-2 and Bcl-xL expression, suggesting that this decrease in hepatocyte apoptosis is mediated through Bcl-2 and Bcl-xL. In summary, SCF and c-kit expression was increased after APAP-induced liver injury. Administration of exogenous SCF reduces mortality in APAP-treated mice, increases hepatocyte proliferation, and prevents hepatocyte apoptosis induced by APAP, suggesting that these molecules are important in the liver's recovery from these injuries.

Genes related to apoptosis predict necrosis of the liver as a phenotype observed in rats exposed to a compendium of hepatotoxicants

Background

Some of the biochemical events that lead to necrosis of the liver are well-known. However, the pathogenesis of necrosis of the liver from exposure to hepatotoxicants is a complex biological response to the injury. We hypothesize that gene expression profiles can serve as a signature to predict the level of necrosis elicited by acute exposure of rats to a variety of hepatotoxicants and postulate that the expression profiles of the predictor genes in the signature can provide insight to some of the biological processes and molecular pathways that may be involved in the manifestation of necrosis of the rat liver.

Results

Rats were treated individually with one of seven known hepatotoxicants and were analyzed for gene expression by microarray. Liver samples were grouped by the level of necrosis exhibited in the tissue. Analysis of significantly differentially expressed genes between adjacent necrosis levels revealed that inflammation follows programmed cell death in response to the agents. Using a Random Forest classifier with feature selection, 21 informative genes were identified which achieved 90%, 80% and 60% prediction accuracies of necrosis against independent test data derived from the livers of rats exposed to acetaminophen, carbon tetrachloride, and allyl alcohol, respectively. Pathway and gene network analyses of the genes in the signature revealed several gene interactions suggestive of apoptosis as a process possibly involved in the manifestation of necrosis of the liver from exposure to the hepatotoxicants. Cytotoxic effects of TNF-α, as well as transcriptional regulation by JUN and TP53, and apoptosis-related genes possibly lead to necrosis.

Conclusion

The data analysis, gene selection and prediction approaches permitted grouping of the classes of rat liver samples exhibiting necrosis to improve the accuracy of predicting the level of necrosis as a phenotypic end-point observed from the exposure. The strategy, along with pathway analysis and gene network reconstruction, led to the identification of 1) expression profiles of genes as a signature of necrosis and 2) perturbed regulatory processes that exhibited biological relevance to the manifestation of necrosis from exposure of rat livers to the compendium of hepatotoxicants.

Human Hepatocytes Can Repopulate Mouse Liver: Histopathology of the Liver in Human Hepatocyte-Transplanted Chimeric Mice and Toxicologic Responses to Acetaminophen

A human hepatocyte-transplanted chimeric mouse has been established by transplantation of human hepatocytes to urokinase-type plasminogen activator transgenic/severe combined immunodeficiency (uPA+/+/SCID) mice. These chimeric mice have various amounts of human hepatocytes that proliferate extensively and progressively replace mouse hepatocytes. In the chimeric liver, hepatic cords and sinusoid-like structures were observed. The human hepatocytes expressed human albumin, human cytochrome P450 enzymes, and human transporter proteins. Furthermore, electron microscopic analysis demonstrated bile canaliculi associated with human hepatocytes in the chimeric mouse livers. These results indicate that the chimeric mouse livers contain functionally intact and differentiated human hepatocytes. Additionally, the toxicologic response of hepatocytes to acetaminophen (APAP) administration was compared in normal and chimeric mouse livers. Following 1,400 mg/kg APAP, mild hepatocellular degeneration was observed in the human hepatocyte areas in the chimeric mice, compared with severe centrilobular hepatocellular necrosis in the ICR mouse livers. In conclusion, these chimeric livers contain functionally differentiated human hepatocytes, and are less susceptible to APAP toxicity, compared to ICR mice.

Mitochondrial bioenergetics after nine-day treatment regimen with benzonidazole in rats

The bioenergetics of cardiac, liver, and kidney mitochondria after 9-day treatment regimen with benzonidazole was studied in rats. The drug was given by oral gavage to adult male Sprague-Dawley rats for 9 consecutive days (100 mg benzonidazole/kg body weight as daily dose). The assayed mitochondrial bioenergetic parameters were the state 4, state 3, respiratory control, efficiency of oxidative phosphorylation, and the activity of the mitochondrial ATP synthase. The results showed that mitochondrial parameters were not altered statistically after in cardiac and kidney mitochondria, but respiratory control in liver mitochondria was statistically increased with benzonidazole treatment. This change was likely due to a slight decrease in state 4 bioenergy metabolism. These results indicate that 9-day benzonidazole treatment regimen had no negative effect on cardiac, liver, and kidney mitochondrial energy metabolism but increased respiratory control in rat liver mitochondria.

Role of JNK translocation to mitochondria leading to inhibition of mitochondria bioenergetics in acetaminophen-induced liver injury

Previously, we demonstrated JNK plays a central role in acetaminophen (APAP)-induced liver injury (Gunawan, B. K., Liu, Z. X., Han, D., Hanawa, N., Gaarde, W. A., and Kaplowitz, N. (2006) Gastroenterology 131, 165-178). In this study, we examine the mechanism involved in activating JNK and explore the downstream targets of JNK important in promoting APAP-induced liver injury in vivo. JNK inhibitor (SP600125) was observed to significantly protect against APAP-induced liver injury. Increased mitochondria-derived reactive oxygen species were implicated in APAP-induced JNK activation based on the following: 1) mitochondrial GSH depletion (maximal at 2 h) caused increased H2O2 release from mitochondria, which preceded JNK activation (maximal at 4 h); 2) treatment of isolated hepatocytes with H2O2 or inhibitors (e.g. antimycin) that cause increased H2O2 release from mitochondria-activated JNK. An important downstream target of JNK following activation was mitochondria based on the following: 1) JNK translocated to mitochondria following activation; 2) JNK inhibitor treatment partially protected against a decline in mitochondria respiration caused by APAP treatment; and 3) addition of purified active JNK to mitochondria isolated from mice treated with APAP plus JNK inhibitor (mitochondria with severe GSH depletion, covalent binding) directly inhibited respiration. Cyclosporin A blocked the inhibitory effect of JNK on mitochondria respiration, suggesting JNK was directly inducing mitochondrial permeability transition in isolated mitochondria from mice treated with APAP plus JNK inhibitor. Addition of JNK to mitochondria isolated from control mice did not affect respiration. Our results suggests that APAP-induced liver injury involves JNK activation, due to increased reactive oxygen species generated by GSH-depleted mitochondria, and translocation of activated JNK to mitochondria where JNK induces mitochondrial permeability transition and inhibits mitochondria bioenergetics.

Disruption of the Smad7 gene enhances CCI4-dependent liver damage and fibrogenesis in mice

Transforming growth factor-beta (TGF-beta) signalling is induced in liver as a consequence of damage and contributes to wound healing with transient activation, whereas it mediates fibrogenesis with long-term up-regulation in chronic disease. Smad-dependent TGF-beta effects are blunted by antagonistic Smad7, which is transcriptionally activated as an immediate early response upon initiation of TGF-beta signalling in most cell types, thereby providing negative feedback regulation. Smad7 can be induced by other cytokines, e.g. IFN-gamma, leading to a crosstalk of these signalling pathways. Here we report on a novel mouse strain, denoted S7DeltaE1, with a deletion of exon I from the endogenous smad7 gene. The mice were viable and exhibited normal adult liver architecture. To obtain insight into Smad7-depend-ent protective effects, chronic liver damage was induced in mice by carbon tetrachloride (CCI4) administration. Subsequent treatment, elevated serum liver enzymes indicated enhanced liver damage in mice lacking functional Smad7. CCI4-dependent Smad2 phosphorylation was pronounced in S7DeltaE1 mice and accompanied by increased numbers of alpha-smooth muscle actin positive 'activated' HSCs. There was evidence for matrix accumulation, with elevated collagen deposition as assessed morphometrically in Sirius red stained tissue and confirmed with higher levels of hydroxyproline in S7DeltaE1 mice. In addition, the number of CD43 positive infiltrating lymphocytes as well as of apoptotic hepatocytes was increased. Studies with primary hepatocytes from S7DeltaE1 and wild-type mice indicate that in the absence of functional Smad7 protein, hepatocytes are more sensitive for TGF-beta effects resulting in enhanced cell death. Furthermore, S7DeltaE1 hepatocytes display increased oxidative stress and cell damage in response to CCI4, as measured by reactive oxygen species production, glutathione depletion, lactate dehydrogenase release and lipid peroxidation. Using an ALK-5 inhibitor all investigated CCI4 effects on hepatocytes were blunted, confirming participation of TGF-beta signalling. We conclude that Smad7 mediates a protective effect from adverse TGF-beta signalling in damaged liver, re-iterating its negative regulatory loop on signalling.

Role of mitochondria in drug-induced cholestatic injury

Mitochondria have multiple functions in eukaryotic cells and are organized into dynamic tubular networks that continuously undergo changes through coordinated fusion and fission and migration through the cytosol. Mitochondria integrate cell-signaling networks, especially those involving the intracellular messenger Ca(2+), into the regulation of metabolic pathways. Recently, it has become clear that mitochondria are central to the three main cell death pathways, namely necrosis, apoptosis, and autophagic cell death. This article discusses the role of mitochondria in drug-induced cholestatic injury to the liver. The role of mitochondria in the cellular adaptation against the toxic effects of bile acids is discussed also.

Serum apoptosis markers in acute liver failure: a pilot study

BACKGROUND & AIMS

We sought to determine whether circulating apoptotic markers are altered in acute liver failure (ALF), differ with etiology, or predict clinical outcome in this condition.

METHODS

Serum levels of soluble Fas (sFas), tumor necrosis factor-alpha (TNF-alpha), hepatocyte growth factor (HGF), and interleukin-6 (IL-6) were measured in 67 acute liver failure patients, as well as controls. In a subset of the groups, we measured serum M-30 antigen, an exposed neoepitope from caspase cleavage. We also assessed M-30 immunoreactivity in liver tissue of ALF patients and controls.

RESULTS

Median levels for TNF-alpha, HGF, IL-6, and M-30 antigen were at least 10-fold greater in ALF than in hepatitis C virus or normal controls (P < .0001). Median day 1 sFas, day 3 sFas, and day 1 HGF levels varied according to etiology of acute liver failure (P = .004, P = .011, and P = .019, respectively), with values for drug-induced liver injury and acetaminophen-related ALF higher than other etiologies. Median M-30 antigen levels were significantly higher in patients who were transplanted and/or died (2183 U/L) than spontaneous survivors (1004 U/L) (P = .026). M-30 immunoreactivity in liver tissue was significantly greater in ALF patients than HCV controls (P = .004).

CONCLUSIONS

TNF-alpha, HGF, IL-6, and M-30 antigen were significantly elevated in ALF. High levels of sFas and HGF might help to confirm a diagnosis of drug-induced liver injury or acetaminophen-related ALF. Higher levels of M-30 antigen are associated with poor clinical outcomes in ALF.

Mitochondrial bax translocation accelerates DNA fragmentation and cell necrosis in a murine model of acetaminophen hepatotoxicity

Mitochondria generate reactive oxygen and peroxynitrite and release endonucleases during acetaminophen (APAP) hepatotoxicity. Because mitochondrial translocation of Bax can initiate these events, we investigated the potential role of Bax in the pathophysiology of hepatic necrosis after 300 mg/kg APAP in fasted C57BL/6 mice. APAP overdose induced Bax translocation from the cytosol to the mitochondria as early as 1 h after APAP injection. At 6 h, there was extensive centrilobular nitrotyrosine staining (indicator for peroxynitrite formation) and nuclear DNA fragmentation. In addition, mitochondrial intermembrane proteins were released into the cytosol. Plasma alanine aminotransferase (ALT) activities of 5610 +/- 600 U/l indicated extensive necrotic cell death. Conversely, Bax gene knockout (Bax(-/-)) mice had 80% lower ALT activities, less DNA fragmentation, and less intermembrane protein release at 6 h. However, immunohistochemical staining for nitrotyrosine or APAP protein adducts did not show differences between wild-type and Bax(-/-) mice. In contrast to the early hepatoprotection in Bax(-/-) mice, plasma ALT activities (7605 +/- 480 U/l) and area of necrosis (53 +/- 6% hepatocytes) in wild-type animals was similar to values in Bax(-/-) mice at 12 h. In addition, there was no difference in DNA fragmentation or nitrotyrosine immunostaining. We concluded that the rapid mitochondrial Bax translocation after APAP overdose has no effect on peroxynitrite formation but that it contributes to the mitochondrial release of proteins, which cause nuclear DNA fragmentation. However, the persistent oxidant stress and peroxynitrite formation in mitochondria may eventually trigger the permeability transition pore opening and release intermembrane proteins independently of Bax.

Mechanisms of drug-induced liver disease

Drug-induced liver injury depends initially on development of hepatocyte stress and cell death, which can be induced directly by parent drugs or by toxic metabolites. Hepatocyte stress can lead to activation of built-in death programs for apoptosis or necrosis. Subsequently, the innate immune system's participation is recruited. The interplay between proinflammatory and anti-inflammatory components of innate immune system determines the outcome of drug-induced liver injury. Both environmental factors and genetic differences in cellular responses to stress and the innate immune response may account for different susceptibilities between individuals to drug-induced liver injury.

Mild hypothermia attenuates liver injury and improves survival in mice with acetaminophen toxicity

BACKGROUND & AIMS

Body temperature may critically affect mechanisms of liver injury in acetaminophen (APAP) hepatotoxicity. In addition, mild hypothermia is used to treat intracranial hypertension in human liver failure without detailed information on its effects on the injured liver itself. Therefore, we investigated the effects of body temperature on the progression of APAP-induced liver injury in mice.

METHODS

Male C57BL6 mice treated with saline or APAP (300 mg/kg intraperitoneally) were maintained at normothermia (35.5-37.5 degrees C) by external warming or were allowed to develop mild hypothermia (32.0-35.0 degrees C) after 2 hours from APAP administration.

RESULTS

Mild hypothermia resulted in improved survival after APAP intoxication. Liver damage was reduced, as assessed histologically and by plasma alanine aminotransferase levels. Early effects of hypothermia included a reduction of hepatic congestion and improved recovery of glycogen stores. At later time points (8-12 hours), APAP-treated mice that were maintained at normothermia manifested increased hepatocyte apoptosis, as assessed by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling staining and cleavage of poly(adenosine diphosphate-ribose) polymerase. Mild hypothermia did not affect the formation of APAP-protein adducts or the depletion of glutathione, nor did it abrogate hepatocyte DNA synthesis.

CONCLUSIONS

Mild hypothermia improved survival and attenuated liver injury and apoptosis in APAP-treated mice by reducing hepatic congestion and improving glycogen recovery without affecting hepatic regeneration. Results of the study underscore the need for a strict control of body temperature in animal models of liver failure and suggest that the benefits of mild hypothermia in liver failure may extend beyond those related to reduced cerebral complications.

Acetaminophen induces ER dependent signaling in mouse liver

Role of endoplasmic reticulum (ER) in liver injury by acetaminophen (AAP) was studied in vivo in mice. Sublethal dose of AAP resulted in a decrease in microsomal total glutathione and in the reduced-to-total glutathione ratio; redox state of thiols of ER resident oxidoreductases ERp72, PDI was shifted towards the oxidized form; ER stress-responsive transcription factor ATF6 was activated. Transcriptional activation and elevated expression of GADD153/CHOP, an ER stress-responsive proapoptotic transcription factor, was observed upon AAP addition. Transient activation of the ER-resident caspase-12 was shown followed by an elevation in procaspase-12 level. Caspase-3 and caspase-8 activation could not be detected. AAP treatment resulted in an increased apoptosis of hepatocytes. Buthionine-sulfoximine treatment was unable to mimic the effects by AAP indicating that glutathione depletion itself is insufficient to provoke apoptosis. The results show that intraluminal redox imbalance of the ER and consequential activation of signaling processes and proapoptotic events are involved in hepatocellular damage caused by AAP overdose.

Gene interaction network analysis suggests differences between high and low doses of acetaminophen

Bayesian networks for quantifying linkages between genes were applied to detect differences in gene expression interaction networks between multiple doses of acetaminophen at multiple time points. Seventeen (17) genes were selected from the gene expression profiles from livers of rats orally exposed to 50, 150 and 1500 mg/kg acetaminophen (APAP) at 6, 24 and 48 h after exposure using a variety of statistical and bioinformatics approaches. The selected genes are related to three biological categories: apoptosis, oxidative stress and other. Gene interaction networks between all 17 genes were identified for the nine dose-time observation points by the TAO-Gen algorithm. Using k-means clustering analysis, the estimated nine networks could be clustered into two consensus networks, the first consisting of the low and middle dose groups, and the second consisting of the high dose. The analysis suggests that the networks could be segregated by doses and were consistent in structure over time of observation within grouped doses. The consensus networks were quantified to calculate the probability distribution for the strength of the linkage between genes connected in the networks. The quantifying analysis showed that, at lower doses, the genes related to the oxidative stress signaling pathway did not interact with the apoptosis-related genes. In contrast, the high-dose network demonstrated significant interactions between the oxidative stress genes and the apoptosis genes and also demonstrated a different network between genes in the oxidative stress pathway. The approaches shown here could provide predictive information to understand high- versus low-dose mechanisms of toxicity.