Early loss of large genomic DNA in vivo with accumulation of Ca2+ in the nucleus during acetaminophen-induced liver injury

The Role of Mechanistic Biomarkers in Understanding Acetaminophen Hepatotoxicity in Humans

Our understanding of the fundamental molecular mechanisms of acetaminophen (APAP) hepatotoxicity began in 1973 to 1974, when investigators at the US National Institutes of Health published seminal studies demonstrating conversion of APAP to a reactive metabolite that depletes glutathione and binds to proteins in the liver in mice after overdose. Since then, additional groundbreaking experiments have demonstrated critical roles for mitochondrial damage, oxidative stress, nuclear DNA fragmentation, and necrotic cell death as well. Over the years, some investigators have also attempted to translate these mechanisms to humans using human specimens from APAP overdose patients. This review presents those studies and summarizes what we have learned about APAP hepatotoxicity in humans so far. Overall, the mechanisms of APAP hepatotoxicity in humans strongly resemble those discovered in experimental mouse and cultured hepatocyte models, and emerging biomarkers also suggest similarities in liver repair. The data not only validate the first mechanistic studies of APAP-induced liver injury performed 50 years ago but also demonstrate the human relevance of numerous studies conducted since then. SIGNIFICANCE STATEMENT: Human studies using novel translational, mechanistic biomarkers have confirmed that the fundamental mechanisms of acetaminophen (APAP) hepatotoxicity discovered in rodent models since 1973 are the same in humans. Importantly, these findings have guided the development and understanding of treatments such as N-acetyl-l-cysteine and 4-methylpyrazole over the years. Additional research may improve not only our understanding of APAP overdose pathophysiology in humans but also our ability to predict and treat serious liver injury in patients.

Central Mechanisms of Acetaminophen Hepatotoxicity: Mitochondrial Dysfunction by Protein Adducts and Oxidant Stress

Acetaminophen (APAP) is an analgesic and antipyretic drug used worldwide, which is safe at therapeutic doses. However, an overdose can induce liver injury and even liver failure. Mechanistic studies in mice beginning with the seminal papers published by B.B. Brodie's group in the 1970s have resulted in important insight into the pathophysiology. Although the metabolic activation of APAP with generation of a reactive metabolite, glutathione depletion, and protein adduct formation are critical initiating events, more recently, mitochondria have come into focus as an important target and decision point of cell death. This review provides a comprehensive overview of the induction of mitochondrial superoxide and peroxynitrite formation and its propagation through a mitogen-activated protein kinase cascade, the mitochondrial permeability transition pore opening caused by iron-catalyzed protein nitration, and the mitochondria-dependent nuclear DNA fragmentation. In addition, the role of adaptive mechanisms that can modulate the pathophysiology, including autophagy, mitophagy, nuclear erythroid 2 p45-related factor 2 activation, and mitochondrial biogenesis, are discussed. Importantly, it is outlined how the mechanisms elucidated in mice translate to human hepatocytes and APAP overdose patients, and how this mechanistic insight explains the mechanism of action of the clinically approved antidote N-acetylcysteine and led to the recent discovery of a novel compound, fomepizole, which is currently under clinical development. SIGNIFICANCE STATEMENT: Acetaminophen (APAP)-induced liver injury is the most frequent cause of acute liver failure in western countries. Extensive mechanistic research over the last several decades has revealed a central role of mitochondria in the pathophysiology of APAP hepatotoxicity. This review article provides a comprehensive discussion of a) mitochondrial protein adducts and oxidative/nitrosative stress, b) mitochondria-regulated nuclear DNA fragmentation, c) adaptive mechanisms to APAP-induced cellular stress, d) translation of cell death mechanisms to overdose patients, and e) mechanism-based antidotes against APAP-induced liver injury.

Inhibiting Hepatocyte Uric Acid Synthesis and Reabsorption Ameliorates Acetaminophen-Induced Acute Liver Injury in Mice

BACKGROUND & AIMS

Acetaminophen (APAP) overdose is the most common cause of drug-induced liver injury worldwide. Uric acid (UA) is involved in sterile inflammation in many organs, but its role in APAP-induced liver injury remains elusive.

METHODS

We quantified the concentration of UA in the serum and liver tissues of APAP-overdosed mice and explored the changes in proteins involved in UA synthesis, absorption, and degeneration on APAP stimulation. We also examined the effects of inhibiting hepatocyte UA synthesis or reabsorption on APAP-induced liver injury in mice. Furthermore, we explored the process of UA clearance by peripheral macrophages.

RESULTS

APAP overdose significantly increased intrahepatic UA contents, which occurred earlier than apparent hepatocyte injury in APAP-overdosed mice. APAP overdose induced significant DNA leakage and may thereby increase the substrate of UA synthesis. APAP overdose also significantly increased the enzymatic activity of xanthine oxidase and urate oxidase and decreased the expression of the UA reabsorption transporter GLUT9 in hepatocytes. Inhibiting hepatocyte UA synthesis by febuxostat or reabsorption by hepatic-specific knockout of GLUT9 alleviated APAP-induced liver injury. Further experiments showed that monosodium urate but not soluble UA may be a major form of UA mediating hepatocyte injury. Additionally, monosodium urate further recruited circulating macrophages into the liver and then aggravated inflammation by increasing the levels of inflammatory factors and reactive oxygen species. Deletion of macrophages significantly ameliorated APAP-induced liver injury in mice.

CONCLUSIONS

APAP overdose induces excessive UA production and leads to local high concentrations in the liver, which further injures cells and induces liver inflammation. Inhibiting the production of UA may be a potential therapeutic option for treating APAP-induced liver injury.

A comprehensive weight of evidence assessment of published acetaminophen genotoxicity data: Implications for its carcinogenic hazard potential

In 2019, the California Office of Environmental Health Hazard Assessment initiated a review of the carcinogenic hazard potential of acetaminophen, including an assessment of its genotoxicity. The objective of this analysis was to inform this review process with a weight-of-evidence assessment of more than 65 acetaminophen genetic toxicology studies that are of widely varying quality and conformance to accepted standards and relevance to humans. In these studies, acetaminophen showed no evidence of induction of point or gene mutations in bacterial and mammalian cell systems or in in vivo studies. In reliable, well-controlled test systems, clastogenic effects were only observed in unstable, p53-deficient cell systems or at toxic and/or excessively high concentrations that adversely affect cellular processes (e.g., mitochondrial respiration) and cause cytotoxicity. Across the studies, there was no clear evidence that acetaminophen causes DNA damage in the absence of toxicity. In well-controlled clinical studies, there was no meaningful evidence of chromosomal damage. Based on this weight-of-evidence assessment, acetaminophen overwhelmingly produces negative results (i.e., is not a genotoxic hazard) in reliable, robust high-weight studies. Its mode of action produces cytotoxic effects before it can induce the stable, genetic damage that would be indicative of a genotoxic or carcinogenic hazard.

Assessment of the biochemical pathways for acetaminophen toxicity: Implications for its carcinogenic hazard potential

In 2019 California's Office of Environmental Health Hazard Assessment (OEHHA) initiated a review of the carcinogenic hazard potential of acetaminophen. In parallel with this review, herein we evaluated the mechanistic data related to the steps and timing of cellular events following therapeutic recommended (≤4 g/day) and higher doses of acetaminophen that may cause hepatotoxicity to evaluate whether these changes indicate that acetaminophen is a carcinogenic hazard. At therapeutic recommended doses, acetaminophen forms limited amounts of N-acetyl-p-benzoquinone-imine (NAPQI) without adverse cellular effects. Following overdoses of acetaminophen, there is potential for more extensive formation of NAPQI and depletion of glutathione, which may result in mitochondrial dysfunction and DNA damage, but only at doses that result in cell death - thus making it implausible for acetaminophen to induce the kind of stable, genetic damage in the nucleus indicative of a genotoxic or carcinogenic hazard in humans. The collective data demonstrate a lack of a plausible mechanism related to carcinogenicity and are consistent with rodent cancer bioassays, epidemiological results reviewed in companion manuscripts in this issue, as well as conclusions of multiple international health authorities.

The development and hepatotoxicity of acetaminophen: reviewing over a century of progress

Acetaminophen (APAP) was first synthesized in the 1800s, and came on the market approximately 65 years ago. Since then, it has become one of the most used drugs in the world. However, it is also a major cause of acute liver failure. Early investigations of the mechanisms of toxicity revealed that cytochrome P450 enzymes catalyze formation of a reactive metabolite in the liver that depletes glutathione and covalently binds to proteins. That work led to the introduction of N-acetylcysteine (NAC) as an antidote for APAP overdose. Subsequent studies identified the reactive metabolite N-acetyl-p-benzoquinone imine, specific P450 enzymes involved, the mechanism of P450-mediated oxidation, and major adducted proteins. Significant gaps remain in our understanding of the mechanisms downstream of metabolism, but several events appear critical. These events include development of an initial oxidative stress, reactive nitrogen formation, altered calcium flux, JNK activation and mitochondrial translocation, inhibition of mitochondrial respiration, the mitochondrial permeability transition, and nuclear DNA fragmentation. Additional research is necessary to complete our knowledge of the toxicity, such as the source of the initial oxidative stress, and to greatly improve our understanding of liver regeneration after APAP overdose. A better understanding of these mechanisms may lead to additional treatment options. Even though NAC is an excellent antidote, its effectiveness is limited to the first 16 hours following overdose.

Hepatoprotective and Antioxidant Capacity of Mallotus repandus Ethyl Acetate Stem Extract against d-Galactosamine-Induced Hepatotoxicity in Rats

Mallotus repandus (M. repandus) is traditionally used to treat muscle pain, itching, fever, rheumatic arthritis, and a variety of liver disorders. The aim of the present work was to evaluate the hepatoprotective activity and the antioxidant potential of the ethyl acetate stem extract of M. repandus (ESMR) against d-galactosamine (d-GalN)-induced hepatopathy, along with a possible mechanism of action in rats. In vivo hepatoprotective activity of ESMR was examined using d-galactosamine (d-GalN)-induced hepatotoxicity in Sprague-Dawley rats. For this purpose, levels of serum diagnostic markers, activity of hepatic antioxidant enzymes, and liver histo-architecture were employed to assess the protective efficacy of ESMR. Furthermore, the total phenolic, flavonoid, and tannin contents were quantitated, and the antioxidant capacity of the extract was evaluated using different methods such as 2,2'-diphenyl-1-picrylhydrazyl (DPPH), nitric oxide (NO), hydrogen peroxide (H2O2), and hydroxyl radical (OH•) scavenging assays. Intraperitoneal d-GalN injection triggered hepatotoxicity, as shown by the noticeable increase in the serum hepatic marker enzymes, bilirubin content, γ-glutamyl transferase (GGT), total cholesterol (TC), triglycerides (TGs), and malondialdehyde (MDA), whereas glutathione, superoxide dismutase, and catalase levels were significantly lower compared with that of the control. Pretreatment with ESMR reduced the hepatic enzyme levels along with bilirubin, GGT, and MDA compared to the d-GalN-intoxicated group. These results were supported by histopathological studies, where d-galactosamine caused coagulative necrosis, hemorrhage, and inflammation. However, pretreatment with ESMR ameliorated the histo-architectural changes and brought them back to normal. Results also revealed that the total polyphenolic, flavonoid, and tannin content, and total antioxidant capacity of ESMR were 136.30 ± 0.78 mg GAE/g mg, 38.72 ± 0.85 mg QE/g, 75.88 ± 0.54 mg TAE/g, and 123.16 ± 0.24 mg AAE/g, respectively. In addition, ESMR inhibited free radicals with IC50 values of 94.47 ± 0.51, 127.33 ± 0.36, 164.12 ± 0.45, and 254.14 ± 0.35 μg/mL in DPPH, NO, H2O2, and OH• free radical scavenging assays, respectively. These findings highlight the protective role of ESMR against hepatic injury induced by d-GalN, which may be attributed to its higher antioxidant properties, thereby scientifically justifying its traditional use.

Emerging and established modes of cell death during acetaminophen-induced liver injury

Acetaminophen (APAP)-induced liver injury is an important clinical and toxicological problem. Understanding the mechanisms and modes of cell death are vital for the development of therapeutic interventions. The histological and clinical features of APAP hepatotoxicity including cell and organelle swelling, karyolysis, and extensive cell contents release lead to the characterization of the cell death as oncotic necrosis. However, the more recent identification of detailed signaling mechanisms of mitochondrial dysfunction, the amplification mechanisms of mitochondrial oxidant stress and peroxynitrite formation by a mitogen-activated protein kinase cascade, mechanisms of the mitochondrial permeability transition pore opening and nuclear DNA fragmentation as well as the characterization of the sterile inflammatory response suggested that the mode of cell death is better termed programmed necrosis. Additional features like mitochondrial Bax translocation and cytochrome c release, mobilization of lysosomal iron and the activation of receptor-interacting protein kinases and the inflammasome raised the question whether other emerging modes of cell death such as apoptosis, necroptosis, ferroptosis and pyroptosis could also play a role. The current review summarizes the key mechanisms of APAP-induced liver injury and compares these with key features of the newly described modes of cell death. Based on the preponderance of experimental and clinical evidence, the mode of APAP-induced cell death should be termed programmed necrosis; despite some overlap with other modes of cell death, APAP hepatotoxicity does not fulfill the characteristics of either apoptosis, necroptosis, ferroptosis, pyroptosis or autophagic cell death.

Mito-tempo protects against acute liver injury but induces limited secondary apoptosis during the late phase of acetaminophen hepatotoxicity

We previously reported that delayed treatment with Mito-tempo (MT), a mitochondria-targeted superoxide dismutase mimetic, protects against the early phase of acetaminophen (APAP) hepatotoxicity by inhibiting peroxynitrite formation. However, whether this protection is sustained to the late phase of toxicity is unknown. To investigate the late protection, C57Bl/6J mice were treated with 300 mg/kg APAP followed by 20 mg/kg MT 1.5 h or 3 h later. We found that both MT treatments protected against the late phase of APAP hepatotoxicity at 12 and 24 h. Surprisingly, MT-treated mice demonstrated a significant increase in apoptotic hepatocytes, while the necrotic phenotype was observed almost exclusively in mice treated with APAP alone. In addition, there was a significant increase in caspase-3 activity and cleavage in the livers of MT-treated mice. Immunostaining for active caspase-3 revealed that the positively stained hepatocytes were exclusively in centrilobular areas. Treatment with the pan-caspase inhibitor ZVD-fmk (10 mg/kg) 2 h post-APAP neutralized this caspase activation and provided additional protection against APAP hepatotoxicity. Treatment with N-acetylcysteine, the current standard of care for APAP poisoning, protected but did not induce this apoptotic phenotype. Mechanistically, MT treatment inhibited APAP-induced RIP3 kinase expression, and RIP3-deficient mice showed caspase activation and apoptotic morphology in hepatocytes analogous to MT treatment. These data suggest that while necrosis is the primary cause of cell death after APAP hepatotoxicity, treatment with the antioxidant MT may switch the mode of cell death to secondary apoptosis in some cells. Modulation of mitochondrial oxidative stress and RIP3 kinase expression play critical roles in this switch.

The role of apoptosis in acetaminophen hepatotoxicity

Although necrosis is recognized as the main mode of cell death induced by acetaminophen (APAP) overdose in animals and humans, more recently an increasing number of publications, especially in the herbal medicine and dietary supplement field, claim an important contribution of apoptotic cell death in the pathophysiology. However, most of these conclusions are based on parameters that are not specific for apoptosis. Therefore, the objective of this review was to re-visit the key signaling events of receptor-mediated apoptosis and APAP-induced programmed necrosis and critically analyze the parameters that are being used as evidence for apoptotic cell death. Both qualitative and quantitative comparisons of parameters such as Bax, Bcl-2, caspase processing and DNA fragmentation in both modes of cell death clearly show fundamental differences between apoptosis and cell death induced by APAP. These observations together with the lack of efficacy of pan-caspase inhibitors in the APAP model strongly supports the conclusion that APAP hepatotoxicity is dominated by necrosis or programmed necrosis and does not involve relevant apoptosis. In order not to create a new controversy, it is important to understand how to use these "apoptosis" parameters and properly interpret the data. These issues are discussed in this review.

Breaking down Leukemia Walls: Heteronemin, a Sesterterpene Derivative, Induces Apoptosis in Leukemia Molt4 Cells through Oxidative Stress, Mitochondrial Dysfunction and Induction of Talin Expression

Heteronemin, the most abundant secondary metabolite in the sponge Hippospongia sp., exhibited potent cytotoxic activity against several cancer cell lines. It increased the percentage of apoptotic cells and reactive oxygen species (ROS) in Molt4 cells. The use of ROS scavenger, N-acetyl cysteine (NAC), suppressed both the production of ROS from mitochondria and cell apoptosis that were induced by heteronemin treatment. Heteronemin upregulated talin and phosphorylated talin expression in Molt4 cells but it only upregulated the expression of phosphorylated talin in HEK293 cells. However, pretreatment with NAC reversed these effects. Talin siRNA reversed the activation of pro-apoptotic cleaved caspases 3 and 9. On the other hand, the downstream proteins including FAK and NF-κB (p65) were not affected. In addition, we confirmed that heteronemin directly modulated phosphorylated talin expression through ROS generation resulting in cell apoptosis, but it did not affect talin/FAK complex. Furthermore, heteronemin interfered with actin microfilament and caused morphology changes. Taken together, these findings suggest that the cytotoxic effect of heteronemin is associated with oxidative stress and induction of phosphorylated talin expression. Our results suggest that heteronemin represents an interesting candidate which can be further developed as a drug lead against leukemia.

DNA Damage Response Regulates Initiation of Liver Regeneration Following Acetaminophen Overdose

Acetaminophen (APAP) overdose is the leading cause of acute liver failure (ALF) with limited treatment options. It is known that liver regeneration following APAP-induced ALF is a deciding factor in the final outcome. Previous studies from our laboratory using an incremental dose model involving a regenerating (300 mg/kg, APAP300) and a nonregenerating (600 mg/kg, APAP600) dose of APAP in mice have revealed several proregenerative pathways that regulate regeneration after APAP overdose. Here we report that DNA damage and repair mechanisms regulate initiation of liver regeneration following APAP overdose. Mice treated with nonregenerating APAP600 dose showed prolonged expression of pH2AX, a marker of the DNA double-strand break (DSB), compared with APAP300. In regenerating APAP300 dose-treated mice, H2AX was rapidly dephosphorylated at Tyr142, indicating timely DNA repair. Expression of several DNA repair proteins was substantially lower with APAP600. Poly(ADP) ribose polymerase (PARP) activation, involved in DNA repair, was significantly higher in the APAP300 group compared to the APAP600 group. Activation of p53, the major cell cycle checkpoint protein, was significantly higher with APAP600 as demonstrated by substantially higher expression of its target genes. Taken together, these data show that massive DNA DSB occurs in high-dose APAP toxicity, and lack of prompt DSB repair after APAP overdose leads to prolonged growth arrest and proliferative senescence, resulting in inhibited liver regeneration.

Hepatoprotective potential of Phyllanthus muellarianus leaf extract: studies on hepatic, oxidative stress and inflammatory biomarkers

CONTEXT

Leaves of Phyllanthus muellarianus (Kuntze) Exell. (Euphorbiacea) are widely used in the management of liver disorders in Nigeria. However, no there is no scientific validation to support this use.

OBJECTIVE

Hepatoprotective effect of Phyllanthus muellarianus aqueous leaf extract was investigated in acetaminophen-induced liver injury mice.

MATERIALS AND METHODS

Hepatoprotective effect of Phyllanthus muellarianus aqueous leaf extract was evaluated in acetaminophen-induced hepatic damage in Swiss albino mice using biomarkers of hepatocellular indices, oxidative stress, proinflammatory factors and lipid peroxidation. Mice received distilled water, 100, 200, or 400 mg/kg b.w of Phyllanthus muellarianus aqueous leaf extract, respectively, for seven days. Treatment groups were challenged with 300 mg/kg b.w of acetaminophen on the sixth day.

RESULTS

Oral administration of Phyllanthus muellarianus aqueous leaf extract significantly (p < 0.05) attenuates acetaminophen-mediated alterations in serum alkaline phosphatase, alanine aminotransferase, aspartate aminotransferase, albumin and total bilirubin by 76.56, 85.41, 89.39, 82.77 and 78.38%. Similarly, acetaminophen-mediated decrease in activities of superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase and glucose 6-phosphate dehydrogenase were significantly attenuated in the liver of mice by 85.10, 80.81, 80.45, 76.23 and 95.22%, respectively. Increased levels of conjugated dienes, lipid hydroperoxides, malondialdehyde, protein carbonyl, fragmented DNA, tumor necrosis factor-α, interleukin-6 and -8 were significantly lowered by Phyllanthus muellarianus aqueous leaf extract.

CONCLUSION

Overall, results of this study show that Phyllanthus muellarianus halted acetaminophen-mediated hepatotoxicity due to its capability to enhance antioxidant enzymes.

Differential susceptibility to acetaminophen-induced liver injury in sub-strains of C57BL/6 mice: 6N versus 6J

Mouse models of acetaminophen (APAP) hepatotoxicity are considered relevant for the human pathophysiology. The C57BL/6 strain is most popular because it is the background strain of gene knock-out mice. However, conflicting results in the literature may have been caused by sub-strain mismatches, e.g. C57BL/6J and C57BL/6N. This study was initiated to determine the mechanism behind the sub-strain susceptibility to APAP toxicity. C57BL/6N and C57BL/6J mice were dosed with 200 mg/kg APAP and sacrificed at different time points. C57BL/6N mice developed significantly more liver injury as measured by plasma ALT activities and histology. Although there was no difference in glutathione depletion or cytochrome P450 activity between groups, C57BL/6N had a higher glutathione disulfide-to-glutathione ratio and more APAP protein adducts. C57BL/6N showed more mitochondrial translocation of phospho-JNK and BAX, and more release of mitochondrial intermembrane proteins apoptosis-inducing factor (AIF), second mitochondria-derived activator of caspases (SMAC), which caused more DNA fragmentation. The increased mitochondrial dysfunction was confirmed in vitro as C57BL/6N hepatocytes had a more precipitous drop in JC-1 fluorescence after APAP exposure.

CONCLUSION

C57BL/6N mice are more susceptible to APAP-induced hepatotoxicity, likely due to increased formation of APAP-protein adducts and a subsequent enhancement of mitochondrial dysfunction associated with aggravated nuclear DNA fragmentation.

Lophirones B and C Attenuate Acetaminophen-Induced Liver Damage in Mice: Studies on Hepatic, Oxidative Stress and Inflammatory Biomarkers

Lophirones B and C are chalcone dimers with proven chemopreventive activity. This study evaluates the hepatoprotective effect lophirones B and C in acetaminophen-induced hepatic damage in mice using biomarkers of hepatocellular indices, oxidative stress, proinflammatory factors and lipid peroxidation. Oral administrations of lophirones B and C significantly (p < 0.05) attenuated acetaminophen-mediated alterations in serum alkaline phosphatase, alanine aminotransferase, aspartate aminotransferase, albumin and total bilirubin. Similarly, acetaminophen-mediated decrease in activities of superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase and glucose 6- phosphate dehydrogenase were significantly attenuated in the liver of mice. Increased levels of conjugated dienes, lipid hydroperoxides, malondialdehyde, protein carbonyl and fragmented DNA were significantly lowered by lophirones B and C. Levels of tumour necrosis factor-α, interleukin-6 and 8 were significantly lowered in serum of acetaminophen treated mice by the chalcone dimers. Overall, results of this study show that lophirones B and C halted acetaminophen-mediated hepatotoxicity.

Mitochondria-targeted antioxidant Mito-Tempo protects against acetaminophen hepatotoxicity

Acetaminophen (APAP) hepatotoxicity is characterized by an extensive mitochondrial oxidant stress. However, its importance as a drug target has not been clarified. To investigate this, fasted C57BL/6J mice were treated with 300 mg/kg APAP and the mitochondria-targeted antioxidant Mito-Tempo (MT) was given 1.5 h later. APAP caused severe liver injury in mice, as indicated by the increase in plasma ALT activities and centrilobular necrosis. MT dose-dependently reduced the injury. Importantly, MT did not affect APAP-protein adducts formation, glutathione depletion or c-jun N-terminal kinase activation and its mitochondrial translocation. In contrast, hepatic glutathione disulfide and peroxynitrite formation were dose-dependently reduced by MT, indicating its effective mitochondrial oxidant stress scavenging capacity. Consequently, mitochondrial translocation of Bax and release of mitochondrial intermembrane proteins such as apoptosis-inducing factor were prevented, and nuclear DNA fragmentation was eliminated. To demonstrate the importance of mitochondria-specific antioxidant property of MT, we compared its efficacy with Tempo, which has the same pharmacological mode of action as MT but lacks the mitochondria targeting moiety. In contrast to the dramatic protection by MT, the same molar dose of Tempo did not significantly reduce APAP hepatotoxicity. In contrast, even a 3 h post-treatment with MT reduced 70 % of the injury, and the combination of MT with N-acetylcysteine (NAC) provided superior protection than NAC alone. We conclude that MT protects against APAP overdose in mice by attenuating the mitochondrial oxidant stress and preventing peroxynitrite formation and the subsequent mitochondrial dysfunction. MT is a promising therapeutic agent for APAP overdose patients.

Acetaminophen from liver to brain: New insights into drug pharmacological action and toxicity

Acetaminophen (APAP) is a well-known analgesic and antipyretic drug. It is considered to be safe when administered within its therapeutic range, but in cases of acute intoxication, hepatotoxicity can occur. APAP overdose is the leading cause of acute liver failure in the northern hemisphere. Historically, studies on APAP toxicity have been focused on liver, with alterations in brain function attributed to secondary effects of acute liver failure. However, in the last decade the pharmacological mechanism of APAP as a cannabinoid system modulator has been documented and some articles have reported "in situ" toxicity by APAP in brain tissue at high doses. Paradoxically, low doses of APAP have been reported to produce the opposite, neuroprotective effects. In this paper we present a comprehensive, up-to-date overview of hepatic toxicity as well as a thorough review of both toxic and beneficial effects of APAP in brain.

Translational biomarkers of acetaminophen-induced acute liver injury

Acetaminophen (APAP) is a commonly used analgesic drug that can cause liver injury, liver necrosis and liver failure. APAP-induced liver injury is associated with glutathione depletion, the formation of APAP protein adducts, the generation of reactive oxygen and nitrogen species and mitochondrial injury. The systems biology omics technologies (transcriptomics, proteomics and metabolomics) have been used to discover potential translational biomarkers of liver injury. The following review provides a summary of the systems biology discovery process, analytical validation of biomarkers and translation of omics biomarkers from the nonclinical to clinical setting in APAP-induced liver injury.

Standardized extract of Vitex doniana Sweet stalls protein oxidation, lipid peroxidation and DNA fragmention in acetaminophen-induced hepatotoxicity

ETHNOPHARMACOLOGICAL RELEVANCE

Vitex doniana fruits are locally used in Nigeria as a remedy in the treatment of jaundice and liver related disease. The effect of methanolic extract of Vitex doniana fruits on acetaminophen induced protein oxidation, lipid peroxidation and DNA fragmentation was investigated in mice.

MATERIALS AND METHODS

Antioxidant activity of the extract (0.2-1.0mg/mL) was investigated in vitro using 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical, superoxide ion, hydrogen peroxide, hydroxyl radical and ferric ion reducing system. Vitex doniana extract at 1.0mg/mL scavenged DPPH, superoxide ion, hydrogen peroxide, and hydroxyl radical by 86%, 78%, 80% and 72% respectively, it also reduced ferric ion significantly. Hepatoprotective effect of Vitex doniana fruits was monitored in acetaminophen-induced hepatotoxicity in mice.

RESULTS

Acetaminophen-mediated alterations in serum alkaline phosphatase, alanine aminotransferase, aspartate aminotransferase, albumin and total bilirubin levels in mice were significantly (P<0.05) attenuated by the extract. Similarly, acetaminophen-mediated decrease in activities of superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase and glucose 6-phosphate dehydrogenase was significantly (P<0.05) attenuated in the liver of mice. Increased levels of conjugated dienes, lipid hydroperoxides, malondialdehyde, protein carbonyl and fragmented DNA were significantly (P<0.05) lowered by methanolic extract of Vitex doniana fruits.

CONCLUSIONS

Overall, the results of this study show that Vitex doniana fruits possess antioxidant properties and halted acetaminophen-mediated oxidative rout on cellular proteins, lipids and DNA, made possible by β-sitosterol, platycodin D, apigenin, saikosaponin, chrysin and ellagitanin in the extract.

Acetaminophen-induced Liver Injury: from Animal Models to Humans

Drug-induced liver injury is an important clinical problem and a challenge for drug development. Whereas progress in understanding rare and unpredictable (idiosyncratic) drug hepatotoxicity is severely hampered by the lack of relevant animal models, enormous insight has been gained in the area of predictable hepatotoxins, in particular acetaminophen-induced liver injury, from a broad range of experimental models. Importantly, mechanisms of toxicity obtained with certain experimental systems, such as in vivo mouse models, primary mouse hepatocytes, and metabolically competent cell lines, are being confirmed in translational studies in patients and in primary human hepatocytes. Despite this progress, suboptimal models are still being used and experimental data can be confusing, leading to controversial conclusions. Therefore, this review attempts to discuss mechanisms of drug hepatotoxicity using the most studied drug acetaminophen as an example. We compare the various experimental models that are used to investigate mechanisms of acetaminophen hepatotoxicity, discuss controversial topics in the mechanisms, and assess how these experimental findings can be translated to the clinic. The success with acetaminophen in demonstrating the clinical relevance of experimental findings could serve as an example for the study of other drug toxicities.

Mechanistic biomarkers in acetaminophen-induced hepatotoxicity and acute liver failure: from preclinical models to patients

INTRODUCTION

Drug hepatotoxicity is a major clinical issue. Acetaminophen (APAP) overdose is especially common. Serum biomarkers used to follow patient progress reflect either liver injury or function, but focus on biomarkers that can provide insight into the basic mechanisms of hepatotoxicity is increasing and enabling us to translate mechanisms of toxicity from animal models into humans.

AREAS COVERED

We review recent advances in mechanistic serum biomarker research in drug hepatotoxicity. Specifically, biomarkers for reactive drug intermediates, mitochondrial dysfunction, nuclear DNA damage, mode of cell death and inflammation are discussed, as well as microRNAs. Emphasis is placed on APAP-induced liver injury.

EXPERT OPINION

Several serum biomarkers of reactive drug intermediates, mitochondrial damage, nuclear DNA damage, apoptosis and necrosis and inflammation have been described. These studies have provided evidence that mitochondrial damage is critical in APAP hepatotoxicity in humans, while apoptosis has only a minor role, and inflammation is important for recovery and regeneration after APAP overdose. Additionally, mechanistic serum biomarkers have been shown to predict outcome as well as, or better than, some clinical scores. In the future, such biomarkers will help determine the need for liver transplantation and, with improved understanding of the human pathophysiology, identify novel therapeutic targets.

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.

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.

Identification of novel translational urinary biomarkers for acetaminophen-induced acute liver injury using proteomic profiling in mice

Drug-induced liver injury (DILI) is the leading cause of acute liver failure. Currently, no adequate predictive biomarkers for DILI are available. This study describes a translational approach using proteomic profiling for the identification of urinary proteins related to acute liver injury induced by acetaminophen (APAP). Mice were given a single intraperitoneal dose of APAP (0–350 mg/kg bw) followed by 24 h urine collection. Doses of ≥275 mg/kg bw APAP resulted in hepatic centrilobular necrosis and significantly elevated plasma alanine aminotransferase (ALT) values (p<0.0001). Proteomic profiling resulted in the identification of 12 differentially excreted proteins in urine of mice with acute liver injury (p<0.001), including superoxide dismutase 1 (SOD1), carbonic anhydrase 3 (CA3) and calmodulin (CaM), as novel biomarkers for APAP-induced liver injury. Urinary levels of SOD1 and CA3 increased with rising plasma ALT levels, but urinary CaM was already present in mice treated with high dose of APAP without elevated plasma ALT levels. Importantly, we showed in human urine after APAP intoxication the presence of SOD1 and CA3, whereas both proteins were absent in control urine samples. Urinary concentrations of CaM were significantly increased and correlated well with plasma APAP concentrations (r = 0.97; p<0.0001) in human APAP intoxicants, who did not present with elevated plasma ALT levels. In conclusion, using this urinary proteomics approach we demonstrate CA3, SOD1 and, most importantly, CaM as potential human biomarkers for APAP-induced liver injury.

The mechanism underlying acetaminophen-induced hepatotoxicity in humans and mice involves mitochondrial damage and nuclear DNA fragmentation

Acetaminophen (APAP) overdose is the predominant cause of acute liver failure in the United States. Toxicity begins with a reactive metabolite that binds to proteins. In rodents, this leads to mitochondrial dysfunction and nuclear DNA fragmentation, resulting in necrotic cell death. While APAP metabolism is similar in humans, the later events resulting in toxicity have not been investigated in patients. In this study, levels of biomarkers of mitochondrial damage (glutamate dehydrogenase [GDH] and mitochondrial DNA [mtDNA]) and nuclear DNA fragments were measured in plasma from APAP-overdose patients. Overdose patients with no or minimal hepatic injury who had normal liver function tests (LTs) (referred to herein as the normal LT group) and healthy volunteers served as controls. Peak GDH activity and mtDNA concentration were increased in plasma from patients with abnormal LT. Peak nuclear DNA fragmentation in the abnormal LT cohort was also increased over that of controls. Parallel studies in mice revealed that these plasma biomarkers correlated well with tissue injury. Caspase-3 activity and cleaved caspase-3 were not detectable in plasma from overdose patients or mice, but were elevated after TNF-induced apoptosis, indicating that APAP overdose does not cause apoptosis. Thus, our results suggest that mitochondrial damage and nuclear DNA fragmentation are likely to be critical events in APAP hepatotoxicity in humans, resulting in necrotic cell death.

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.

Acetaminophen hepatotoxicity and repair: the role of sterile inflammation and innate immunity

Acetaminophen (APAP) hepatotoxicity because of overdose is the most frequent cause of acute liver failure in the western world. Metabolic activation of APAP and protein adduct formation, mitochondrial dysfunction, oxidant stress, peroxynitrite formation and nuclear DNA fragmentation are critical intracellular events in hepatocytes. However, the early cell necrosis causes the release of a number of mediators such as high-mobility group box 1 protein, DNA fragments, heat shock proteins (HSPs) and others (collectively named damage-associated molecular patterns), which can be recognized by toll-like receptors on macrophages, and leads to their activation with cytokine and chemokine formation. Although pro-inflammatory mediators recruit inflammatory cells (neutrophils, monocytes) into the liver, neither the infiltrating cells nor the activated resident macrophages cause any direct cytotoxicity. In contrast, pro- and anti-inflammatory cytokines and chemokines can directly promote intracellular injury mechanisms by inducing nitric oxide synthase or inhibit cell death mechanisms by the expression of acute-phase proteins (HSPs, heme oxygenase-1) and promote hepatocyte proliferation. In addition, the newly recruited macrophages (M2) and potentially neutrophils are involved in the removal of necrotic cell debris in preparation for tissue repair and resolution of the inflammatory response. Thus, as discussed in detail in this review, the preponderance of experimental evidence suggests that the extensive sterile inflammatory response during APAP hepatotoxicity is predominantly beneficial by limiting the formation and the impact of pro-inflammatory mediators and by promoting tissue repair.

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.

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.

c-Jun N-terminal kinase modulates oxidant stress and peroxynitrite formation independent of inducible nitric oxide synthase in acetaminophen hepatotoxicity

Acetaminophen (APAP) overdose, which causes liver injury in animals and humans, activates c-jun N-terminal kinase (JNK). Although it was shown that the JNK inhibitor SP600125 effectively reduced APAP hepatotoxicity, the mechanisms of protection remain unclear. C57Bl/6 mice were treated with 10mg/kg SP600125 or vehicle (8% dimethylsulfoxide) 1h before 600mg/kg APAP administration. APAP time-dependently induced JNK activation (detected by JNK phosphorylation). SP600125, but not the vehicle, reduced JNK activation, attenuated mitochondrial Bax translocation and prevented the mitochondrial release of apoptosis-inducing factor at 4-12h. Nuclear DNA fragmentation, nitrotyrosine staining, tissue GSSG levels and liver injury (plasma ALT release and necrosis) were partially attenuated by the vehicle (-65%) and completely eliminated by SP600125 (-98%) at 6 and 12h. Furthermore, SP600125 attenuated the increase of inducible nitric oxide synthase (iNOS) mRNA and protein. However, APAP did not enhance plasma nitrite+nitrate levels (NO formation); SP600125 had no effect on this parameter. The iNOS inhibitor L-NIL did not reduce NO formation or injury after APAP but prevented NO formation caused by endotoxin. Since SP600125 completely eliminated the increase in hepatic GSSG levels, an indicator of mitochondrial oxidant stress, it is concluded that the inhibition of peroxynitrite was mainly caused by reduced superoxide formation. Our data suggest that the JNK inhibitor SP600125 protects against APAP-induced liver injury in part by attenuation of mitochondrial Bax translocation but mainly by preventing mitochondrial oxidant stress and peroxynitrite formation and thereby preventing the mitochondrial permeability transition pore opening, a key event in APAP-induced cell necrosis.

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.

Acute exposure to ozone exacerbates acetaminophen-induced liver injury in mice

Ozone (O(3)), an oxidant air pollutant in photochemical smog, principally targets epithelial cells lining the respiratory tract. However, changes in gene expression have also been reported in livers of O(3)-exposed mice. The principal aim of the present study was to determine if acute exposure to environmentally relevant concentrations of O(3) could cause exacerbation of drug-induced liver injury in mice. Overdose with acetaminophen (APAP) is the most common cause of drug-induced liver injury in developed countries. In the present study, we examined the hepatic effects of acute O(3) exposure in mice pretreated with a hepatotoxic dose of APAP. C57BL/6 male mice were fasted overnight and then given APAP (300 mg/kg ip) or saline vehicle (0 mg/kg APAP). Two hours later, mice were exposed to 0, 0.25, or 0.5 ppm O(3) for 6 h and then sacrificed 9 or 32 h after APAP administration (1 or 24 h after O(3) exposure, respectively). Animals euthanized at 32 h were given 5-bromo-2-deoxyuridine 2 h before sacrifice to identify hepatocytes undergoing reparative DNA synthesis. Saline-treated mice exposed to either air or O(3) had no liver injury. All APAP-treated mice developed marked centrilobular hepatocellular necrosis that increased in severity with time after APAP exposure. O(3) exposure increased the severity of APAP-induced liver injury as indicated by an increase in necrotic hepatic tissue and plasma alanine aminotransferase activity. O(3) also caused an increase in neutrophil accumulation in livers of APAP-treated animals. APAP induced a 10-fold increase in the number of bromodeoxyuridine-labeled hepatocytes that was markedly attenuated by O(3) exposure. Gene expression analysis 9 h after APAP revealed differential expression of genes involved in inflammation, oxidative stress, and cellular regeneration in mice treated with APAP and O(3) compared to APAP or O(3) alone, providing some indications of the mechanisms behind the APAP and O(3) potentiation. These results suggest that acute exposure to near ambient concentrations of this oxidant air pollutant may exacerbate drug-induced liver injury by delaying hepatic repair.

Mechanism of protection by metallothionein against acetaminophen hepatotoxicity

Acetaminophen (APAP) overdose is the most frequent cause of drug-induced liver failure in the US. Metallothionein (MT) expression attenuates APAP-induced liver injury. However, the mechanism of this protection remains incompletely understood. To address this issue, C57BL/6 mice were treated with 100 micromol/kg ZnCl2 for 3 days to induce MT. Twenty-four hours after the last dose of zinc, the animals received 300 mg/kg APAP. Liver injury (plasma ALT activities, area of necrosis), DNA fragmentation, peroxynitrite formation (nitrotyrosine staining), MT expression, hepatic glutathione (GSH), and glutathione disulfide (GSSG) levels were determined after 6 h. APAP alone caused severe liver injury with oxidant stress (increased GSSG levels), peroxynitrite formation, and DNA fragmentation, all of which were attenuated by zinc-induced MT expression. In contrast, MT knockout mice were not protected by zinc. Hydrogen peroxide-induced cell injury in primary hepatocytes was dependent only on the intracellular GSH levels but not on MT expression. Thus, the protective effect of MT in vivo was not due to the direct scavenging of reactive oxygen species. Zinc treatment had no effect on the early GSH depletion kinetics after APAP administration, which is an indicator of the metabolic activation of APAP to its reactive metabolite N-acetyl-p-benzoquinone imine (NAPQI). However, MT was able to effectively trap NAPQI by covalent binding. We conclude that MT scavenges some of the excess NAPQI after GSH depletion and prevents covalent binding to cellular proteins, which is the trigger for the propagation of the cell injury mechanisms through mitochondrial dysfunction and nuclear DNA damage.

Antihyperlipidemic effect of carvacrol on D-galactosamine-induced hepatotoxic rats

Carvacrol (2-methyl-5-(1-methylethyl)-phenol) is a predominant monoterpenic phenol occuring in many essential oils of the family Labiatae including, Origanum, Satureja, Thymbra, Thymus, and Corydothymus species. The present study was designed to investigate the effect of carvacrol on D-galactosamine (D-GalN)-induced hepatotoxicity in rats. D-GalN-hepatotoxic rats exhibited elevation in the serum bilirubin level and the activities of the hepatic marker enzymes aspartate aminotransferase, alanine aminotransferase, alkaline phosphatase, and gamma glutamyl transpeptidase. In the plasma, increased levels of very low density lipoprotein cholesterol and low density lipoprotein cholesterol and decreased high density lipoprotein cholesterol were observed. Further, an increase in the levels of total cholesterol, phospholipids, triglycerides, and free fatty acids in the plasma and tissues of liver and kidney were observed in hepatotoxic rats. The administration of carvacrol for 21 days prevented and improved these parameters toward normalcy. The results suggest that carvacrol affords a significant hepatoprotective and hypolipidemic effect against D-GalN-induced-rats.

Protective effect of a phytocompound on oxidative stress and DNA fragmentation against paracetamol-induced liver damage

The hepatoprotective potential DTS (1.5 g/kg bw, Denshici-to-Chiusei, Kyotsu Jigyo, Tokyo, Japan) was evaluated against either toxic (1.5 g/kg bw) and sub-toxic (150 mg/kg bw) dosage of paracetamol-induced liver injury in Sprague-Dawley rat. Paracetamol intoxication caused a reduction of serum total protein and increase levels of serum alkaline phosphatase (ALP), aspartate aminotranferase (AST) and serum alanine aminotranferase (ALT) at higher extent in the toxic group. This phenomenon was paralleled by an impaired liver redox status (reduced glutathione (GSH), superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase (CAT) and increased MDA in both paracetamol-administered groups. Moreover, a marked reduction of ATPase and thiols together with DNA fragmentation occurred in liver tissue. Animals pretreated with DTS showed a marked mitigation of the severity of liver enzyme and of the impaired redox status of the liver. Moreover, DTS partly prevented the DNA fragmentation and the decline of liver tissue ATPase and protein thiol assay as compared with both groups treated with paracetamol alone. Although more detailed studies are awaited to ascertain the detailed mode of action of DTS, it wouls seem to be related to the prevention of formation of the reactive oxygen groups thereby preventing the damage on the hepatocytes and possibly modulating the genes responsible for synthesis of liver antioxidant enzymes thus providing marked DNA protection.<br />

Protection of acetaminophen induced mitochondrial dysfunctions and hepatic necrosis via Akt-NF-kappaB pathway: role of a novel plant protein

Oxidative stress is a major cause of drug induced hepatic diseases. The present study aims to investigate the antioxidative signaling mechanism of a protein isolated from the herb, Cajanus indicus against acetaminophen induced necrotic cell death. We found that incubation of hepatocytes with the protein prevented acetaminophen-induced loss in cell viability, reduction in glutathione level and enhancement of reactive oxygen species generation. Treatment of mice with the protein before administration of acetaminophen also reduced serum nitrite and TNF-alpha formation. Moreover, it counteracted acetaminophen-induced loss in mitochondrial membrane potential, loss in adenosine tri phosphate and rise in intracellular calcium. Investigating the cell signaling pathways, we found that the protein exerts its protective action via the activation of NF-kappaB and Akt and deactivation of STAT-1. Surprisingly, no role of ERK1/2 or STAT-3 was found in the protein-mediated protection of hepatocytes during acetaminophen exposure. Finally, we found that acetaminophen introduces necrosis as the primary phenomena of cell death and protein treatment decreased the necrotic process as evident from the DNA fragmentation and flow-cytometry studies. In addition, administration of the protein to mice before acetaminophen application showed fewer number of TUNEL positive cells. Combining, data suggest that the protein possesses cytoprotective activity against acetaminophen-induced oxidative cellular damage and prevents hepatocytes from necrotic death.

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.

Chromatin breakdown by deoxyribonuclease1 promotes acetaminophen-induced liver necrosis: an ultrastructural and histochemical study on male CD-1 mice

We analyzed in male wild-type (WT) and Dnase1 knockout (KO) CD-1 mice after acetaminophen (APAP)-intoxication the hepatolobular distribution of APAP-adducts in relation to DNA-damage by terminal deoxyribonucleotidyl-transferase dUTP nick end-labeling (TUNEL), the ultrastructural alterations of hepatocellular morphology and the intracellular localization of Dnase1. Treatment of WT-mice with 600 mg/kg APAP led to extensive pericentral necrosis. Electron microscopy (EM) demonstrated vesiculation of the rough endoplasmatic reticulum and swelling of mitochondria. Pericentral WT-hepatocyte nuclei exhibited pyknosis, karyorrhexis and karyolysis. In contrast, livers from treated KO-mice exhibited almost normal light microscopical structure and EM revealed only mild signs of hepatocellular damage. In WT-mice several layers of pericentral hepatocytes displayed APAP-adduct formation and subsequent DNA-damage, whereas in KO-animals only few cells were affected. Serum aminotransferases rose similarly in both mouse strains up to 12 h, thereafter increased only in WT-mice. Immunogold-EM revealed the translocation of Dnase1 from the rER into the nuclei of treated WT-mice. In KO-mice, APAP-adduct formation was retarded and less extensive suggesting that detoxification of APAP must have been more effective in KO-mice possibly due to the lack of energy depletion otherwise caused by Dnase1-induced DNA-damage in WT-mice.

Efficacy of Sargassum polycystum (Phaeophyceae) sulphated polysaccharide against paracetamol-induced DNA fragmentation and modulation of membrane-bound phosphatases during toxic hepatitis

1. The aim of the present study was to assess the protective effect of Sargassum polycystum (sulphated polysaccharide) extract against paracetamol-induced DNA strand breaks and modulation of membrane-bound phosphatases, protein thiols and inorganic cations during toxic hepatitis. 2. Seaweed extract (200 mg/kg per day for 21 days) was administered to male Wistar rats against paracetamol challenge. Serum and liver tissues were used to assess levels of ATPase, protein thiols and inorganic cations using atomic absorption spectroscopy. The fragmentation of DNA was assessed by agarose gel electrophoresis. 3. Paracetamol induced intracellular stress, accompanied by changes in the structural and functional characteristics of liver cell membranes, which affected DNA integrity, membrane-bound ATPase and inorganic cations homeostasis. Rats intoxicated with paracetamol (800 mg/kg, i.p.) showed significant impairment in activities of total ATPase, Mg2+-ATPase, Ca+-ATPase and Na+/K+-ATPase, with concomitant changes in the levels of tissue protein thiols and inorganic cations, such as Na+, K+ and Ca2+. These changes were prevented in animals pretreated with S. polycystum extract, which indicates that S. polycystum supplementation could exert some protective effect against paracetamol-induced toxic hepatitis in rats. 4. The protective effect of the seaweed extract may be due to the presence of sulphated compounds that have free radical-scavenging activity.

Role of inflammation in the mechanism of acetaminophen-induced hepatotoxicity

Acetaminophen (AAP) overdose and the resulting hepatotoxicity is an important clinical problem. In addition, AAP is widely used as a prototype hepatotoxin to study mechanisms of chemical-induced cell injury and to test the hepatoprotective potential of new drugs and herbal medicines. Because of its importance, the mechanisms of AAP-induced liver cell injury have been extensively investigated and controversially discussed for > 30 years. This review highlights recent new insight into intracellular events critical for liver cell death. In addition, the relevance of the inflammatory response is addressed, including cytotoxic and inflammatory mediators generated by activated inflammatory cells, that is, resident macrophages and lymphocytes as well as newly recruited blood-derived leukocytes. Inflammation is a critical component of the overall pathophysiology, not only as a potential factor that may aggravate cell damage, but more importantly as a vital response to limit cell injury, remove cell debris and promote regeneration.

Deoxyribonuclease 1 aggravates acetaminophen-induced liver necrosis in male CD-1 mice

An overdose of acetaminophen (APAP) (N-acetyl-p-aminophenol) leads to hepatocellular necrosis induced by its metabolite N-acetyl-p-benzoquinone-imine, which is generated during the metabolic phase of liver intoxication. It has been reported that DNA damage occurs during the toxic phase; however, the nucleases responsible for this effect are unknown. In this study, we analyzed the participation of the hepatic endonuclease deoxyribonuclease 1 (DNASE1) during APAP-induced hepatotoxicity by employing a Dnase1 knockout (KO) mouse model. Male CD-1 Dnase1 wild-type (WT) (Dnase1+/+) and KO (Dnase1-/-) mice were treated with 2 different doses of APAP. Hepatic histopathology was performed, and biochemical parameters for APAP metabolism and necrosis were investigated, including depletion of glutathione/glutathione-disulfide (GSH+GSSG), beta-nicotinamide adenine dinucleotide (NADH+NAD+), and adenosine triphosphate (ATP); release of aminotransferases and Dnase1; and occurrence of DNA fragmentation. As expected, an APAP overdose in WT mice led to massive hepatocellular necrosis characterized by the release of aminotransferases and depletion of hepatocellular GSH+GSSG, NADH+NAD+, and ATP. These metabolic events were accompanied by extensive DNA degradation. In contrast, Dnase1 KO mice were considerably less affected. In conclusion, whereas the innermost pericentral hepatocytes of both mouse strains underwent necrosis to the same extent independent of DNA damage, the progression of necrosis to more outwardly located cells was dependent on DNA damage and only occurred in WT mice. Dnase1 aggravates APAP-induced liver necrosis.

Role of caspases in acetaminophen-induced liver injury

The mode of cell death after acetaminophen (AAP) overdose is controversially discussed. A recent study reported a protective effect of the pancaspase inhibitor Z-VAD-fmk against AAP toxicity in vivo but the mechanism of protection remained unclear. Therefore, the objective of this investigation was to assess if Z-VAD-fmk or the low doses of dimethyl sulfoxide (DMSO) used as solvent were responsible for the protection. Treatment with 10 mg/kg Z-VAD-fmk or diluted DMSO (0.25 ml/kg) for 15 min before but not 2.5 h after AAP prevented the oxidant stress (hepatic glutathione disulfide content; nitrotyrosine staining), DNA fragmentation (anti-histone ELISA, TUNEL assay) and liver injury (plasma ALT activities) at 6 h after administration of 300 mg/kg AAP. Even a lower dose (0.1 ml/kg) of DMSO was partially effective. DMSO pretreatment also attenuated the initial decline in hepatic glutathione levels. On the other hand, 10 microM Z-VAD-fmk was unable to prevent AAP-induced cell death in primary cultured mouse hepatocytes. We conclude that Z-VAD-fmk does not protect against AAP-induced liver injury and, therefore, caspases are not involved in the mechanism of AAP-induced liver injury. In contrast, the protection in vivo is caused by the diluted DMSO, which is used to solubilize the inhibitor Z-VAD-fmk. The results emphasize that even very low doses of DMSO, which are generally necessary to dissolve water-insoluble inhibitors, can have a profound impact on the toxicity of drugs and chemicals when metabolic activation is a critical aspect of the mechanism of cell injury.

Intracellular signaling mechanisms of acetaminophen-induced liver cell death

Acetaminophen hepatotoxicity is the leading cause of drug-induced liver failure. Despite substantial efforts in the past, the mechanisms of acetaminophen-induced liver cell injury are still incompletely understood. Recent advances suggest that reactive metabolite formation, glutathione depletion, and alkylation of proteins, especially mitochondrial proteins, are critical initiating events for the toxicity. Bcl-2 family members Bax and Bid then form pores in the outer mitochondrial membrane and release intermembrane proteins, e.g., apoptosis-inducing factor (AIF) and endonuclease G, which then translocate to the nucleus and initiate chromatin condensation and DNA fragmentation, respectively. Mitochondrial dysfunction, due to covalent binding, leads to formation of reactive oxygen and peroxynitrite, which trigger the membrane permeability transition and the collapse of the mitochondrial membrane potential. In addition to the diminishing capacity to synthesize ATP, endonuclease G and AIF are further released. Endonuclease G, together with an activated nuclear Ca2+,Mg2+-dependent endonuclease, cause DNA degradation, thereby preventing cell recovery and regeneration. Disruption of the Ca2+ homeostasis also leads to activation of intracellular proteases, e.g., calpains, which can proteolytically cleave structural proteins. Thus, multiple events including massive mitochondrial dysfunction and ATP depletion, extensive DNA fragmentation, and modification of intracellular proteins contribute to the development of oncotic necrotic cell death in the liver after acetaminophen overdose. Based on the recognition of the temporal sequence and interdependency of these mechanisms, it appears most promising to therapeutically target either the initiating event (metabolic activation) or the central propagating event (mitochondrial dysfunction and peroxynitrite formation) to prevent acetaminophen-induced liver cell death.

Peroxynitrite-induced mitochondrial and endonuclease-mediated nuclear DNA damage in acetaminophen hepatotoxicity

Intracellular sources of peroxynitrite formation and potential targets for this powerful oxidant and nitrating agent have not been identified after acetaminophen (AAP) overdose. Therefore, we tested the hypothesis that peroxynitrite generated in mitochondria may be responsible for mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) damage. C3Heb/FeJ mice were treated with 300 mg/kg AAP and monitored for up to 12 h. Loss of mtDNA (assayed by slot blot hybridization) and substantial nDNA fragmentation (evaluated by anti-histone enzyme-linked immunosorbent assay, terminal deoxynucleotidyl transferase dUTP nick-end labeling assay, and agarose gel electrophoresis) were observed as early as 3 h after AAP overdose. Analysis of nitrotyrosine protein adducts in subcellular fractions established that peroxynitrite was generated predominantly in mitochondria beginning at 1 h after AAP injection. Delayed treatment with a bolus dose of glutathione (GSH) accelerated the recovery of mitochondrial glutathione, which then effectively scavenged peroxynitrite. However, mtDNA loss was only partially prevented. Despite the absence of nitrotyrosine adducts in the nucleus after AAP overdose, nDNA damage was almost completely eliminated with GSH administration. A direct comparison of nDNA damage after AAP overdose with nDNA fragmentation during tumor necrosis factor receptor-mediated apoptosis showed similar DNA ladders on agarose gels but quantitatively different results in three other assays. We conclude that peroxynitrite may be partially responsible for mtDNA loss but is not directly involved in nDNA damage. In contrast, nDNA fragmentation after AAP overdose is not caused by caspase-activated DNase but most likely by other intracellular DNase(s), whose activation is dependent on the mitochondrial oxidant stress and peroxynitrite formation.

Effect of Sargassum polycystum (Phaeophyceae)-sulphated polysaccharide extract against acetaminophen-induced hyperlipidemia during toxic hepatitis in experimental rats

The effect of Sargassum polycystum crude extract on lipid metabolism was examined against acetaminophen-induced (800 mg/kg body wt., intraperitoneally) hyperlipidemia during toxic hepatitis in experimental rats. The animals intoxicated with acetaminophen showed significant elevation in the levels of cholesterol, triglycerides and free fatty acid in both serum and liver tissue. The levels of tissue total lipids and serum LDL-cholesterol were also elevated with depleted levels of serum HDL-cholesterol and tissue phospholipid. The acetaminophen-induced animals showed significant alterations in the activities of lipid metabolizing enzymes serum lecithin cholesterol acyl transferase (LCAT) and hepatic triglyceride lipase (HTGL). The levels of liver tissue fatty acids (saturated, mono and polyunsaturated) such as palmitic acid, stearic acid, oleic acid, linoleic acid, arachidonic acid and linolenic acid monitored by gas chromatography were considerably altered in acetaminophen intoxicated animals when compared with control animals. The prior oral administration of Sargassum polycystum (200 mg/kg body wt./day for a period of 15 days) crude extract showed considerable prevention in the severe disturbances of lipid profile and metabolizing enzymes triggered by acetaminophen during hepatic injury. Liver histology also showed convincing supportive evidence regarding their protective nature against fatty changes induced during acetaminophen intoxication. Thus the present study indicates that the protective nature of Sargassum polycystum extract may be due to the presence of active compounds possessing antilipemic property against acetaminophen challenge.

Oxidative stress mediates drug-induced hepatotoxicity in rats: a possible role of DNA fragmentation

Sodium diethyldithiocarbamate, diclofenac and ketoconazol are three important chemotherapeutic agents that are commonly associated with hepatotoxicity. This study was undertaken to provide a better understanding of the mechanism through which these drugs induce hepatotoxicity. Some of the possible mechanisms underlying such modulation were investigated. The hepatotoxic activity of sodium diethyldithiocarbamate (800 mg kg(-1)); diclofenac (200 mg kg(-1)) and ketoconazol (100 mg kg(-1)) were investigated in vivo through the assessment of liver functions, lipid peroxidation and histopathological examination. It was found that all drugs have induced severe hepatic damage as evidenced by the elevation serum aminotransferase activities and confirmed by histological changes of liver. In addition, the drug-induced hepatotoxicity was also associated with massive liver DNA fragmentation and an increase in lipid peroxidation. These results strongly suggest a positive correlation between hepatotoxicity and DNA fragmentation. Moreover, this study also implicates calcium as a potential mediator of the drug-induced oxidative stress associated with hepatotoxicity.

Glycogen Synthase Kinase-3 Mediates Acetaminophen-Induced Apoptosis in Human Hepatoma Cells

The mild analgesic drug acetaminophen (AAP) induces severe hepatic injury when taken at excessive doses. Recent evidence shows that the initial form of damage is through apoptosis, but this fails to go to completion and degenerates into necrosis. The aim of this study was to elucidate the mechanism through which AAP induces apoptosis using human HuH7 hepatoma cells as an in vitro model system to investigate the initial phase of AAP-induced hepatic injury. AAP-induced apoptosis in HuH7 cells as evidenced by chromatin condensation was preceded by the translocation of Bax to mitochondria and the cytoplasmic release of the proapoptotic factors cytochrome c and Smac/DIABLO. A concomitant loss of mitochondrial membrane potential occurred. Activation of the mitochondrial pathway of apoptosis led to the activation of execution caspases-3 and -7. AAP-induced apoptosis and cell death was blocked by inhibitors of caspases but not by inhibitors of calpains, cathepsins, and serine proteases. Apoptosis was unaffected by inhibitors of the mitochondrial permeability transition pore and by inhibitors of Jun NH(2)-terminal kinases, p38 mitogen-activated protein kinase, or mitogen-activated protein kinase kinase 1/2. However, pharmacological inhibition of glycogen synthase kinase-3 (GSK-3) delayed and decreased the extent of AAP-induced apoptosis. In comparison, endoplasmic reticulum stress-induced but not prooxidant-induced apoptosis of HuH7 cells was sensitive to GSK-3 inhibition. It is concluded that AAP-induced apoptosis involves the mitochondrial pathway of apoptosis that is mediated by GSK-3 and most likely initiated through an endoplasmic reticulum stress response.

Protective effect of Premna tomentosa (L. Verbenaceae) extract on membrane-bound phosphatases and inorganic cations transport in acetaminophen-induced hepatotoxicity rats

Hepatic injury elicits intracellular stress that leads to peroxidation of membrane lipids accompanied by alteration of structural and functional characteristics of membrane, which affect the activities of membrane-bound ATPases. The present study appraised the membrane protective effect of Premna tomentosa, a hepatoprotective drug used in Indian traditional medicine. Wistar strain rats were pre-treated with Premna tomentosa extract (750 mg/kg, orally) for 15 days, 24 h prior to administration of acetaminophen (640 mg/kg, orally). During acetaminophen intoxication, the levels of membrane-bound enzymes were significantly decreased, total ATPase (1.63-fold), Mg(2+)ATPase (1.9-fold), Ca(2+)ATPase (1.33-fold) and Na(+)K(+)ATPase (1.73-fold) which was accompanied by changes in the levels of inorganic cations N+, K+ and Ca2+. These alterations were prevented by Premna tomentosa extract pre-treatment, which shows that Premna tomentosa supplementation could exert a beneficial effect against liver injury-induced membrane damage. The potential of the plant might be credited to the presence of antioxidant compound limonene in the plant.