Inhibitor of apoptosis signal-regulating kinase 1 protects against acetaminophen-induced liver injury

ASK1/ p38 axis inhibition blocks the release of mitochondrial "danger signals" from hepatocytes and suppresses progression to cirrhosis and liver cancer

BACKGROUND AND AIMS

Apoptosis Signal-regulating Kinase 1 (ASK1) is activated by various pathological stimuli and induces cell apoptosis through downstream p38 activation. We studied the effect of pharmacological ASK1 inhibition on cirrhosis and its sequelae using comprehensive preclinical in vivo and in vitro systems.

APPROACH AND RESULTS

Short-term (4-6 wk) and long-term (24-44 wk) ASK1 inhibition using small molecule GS-444217 was tested in thioacetamide-induced and BALB/c. Mdr2-/- murine models of cirrhosis and HCC, and in vitro using primary hepatocyte cell death assays. Short-term GS-444217 therapy in both models strongly reduced phosphorylated p38, hepatocyte death, and fibrosis by up to 50%. Profibrogenic release of mitochondrial DAMP mitochondrial deoxyribonucleic acid from dying hepatocytes was blocked by ASK1 or p38 inhibition. Long-term (24 wk) therapy in BALBc.Mdr2 - / - model resulted in a moderate 25% reduction in bridging fibrosis, but not in net collagen deposition. Despite this, the development of cirrhosis was effectively prevented, with strongly reduced p21 + hepatocyte staining (by 72%), serum ammonia levels (by 46%), and portal pressure (average 6.07 vs. 8.53 mm Hg in controls). Extended ASK1 inhibition for 44 wk in aged BALB/c. Mdr2-/- mice resulted in markedly reduced tumor number and size by ~50% compared to the control group.

CONCLUSIONS

ASK1 inhibition suppresses the profibrogenic release of mitochondrial deoxyribonucleic acid from dying hepatocytes in a p38-dependent manner and protects from liver fibrosis. Long-term ASK1 targeting resulted in diminished net antifibrotic effect, but the progression to liver cirrhosis and cancer in BALBc/ Mdr2- / - mice was effectively inhibited. These data support the clinical evaluation of ASK1 inhibitors in fibrotic liver diseases.

Exploring novel indazole derivatives as ASK1 inhibitors: Design, synthesis, biological evaluation and docking studies

Apoptosis signal regulated kinase 1 (ASK1, MAP3K5) is a member of the mitogen activated protein kinase (MAPK) signaling pathway, involved in cell survival, differentiation, stress response, and apoptosis. ASK1 kinase inhibition has become a promising strategy for the treatment of Non-alcoholic steatohepatitis (NASH) disease. A series of novel ASK1 inhibitors with indazole scaffolds were designed and synthesized, and their ASK1 kinase activities were evaluated. The System Structure Activity Relationship (SAR) study discovered a promising compound 33c, which has a strong inhibitory effect on ASK1. Noteworthy observations included a discernible reduction in lipid droplets within LO2 cells stained with Oil Red O, coupled with a decrease in LDL, CHO, and TG content within the NASH model cell group. Mechanistic inquiries revealed that compound 33c could inhibit the protein expression levels of the upregulated ASK1-p38/JNK signaling pathway in TNF-α treated HGC-27 cells and regulate apoptotic proteins. In summary, these findings suggest that compound 33c may be valuable for further research as a potential candidate compound against NASH.

Kaempferol sophoroside glucoside mitigates acetaminophen-induced hepatotoxicity: Role of Nrf2/NF-κB and JNK/ASK-1 signaling pathways

APAP (Acetaminophen)-induced hepatic injury is a major public health threat that requires continuous searching for new effective therapeutics. KSG (Kaempferol-3-sophoroside-7-glucoside) is a kaempferol derivative that was separated from plant species belonging to different genera. This study explored the protective effects of KSG on ALI (acute liver injury) caused by APAP overdose in mice and elucidated its possible mechanisms. The results showed that KSG pretreatment alleviated APAP-induced hepatic damage as it reduced hepatic pathological lesions as well as the serum parameters of liver injury. Moreover, KSG opposed APAP-associated oxidative stress and augmented hepatic antioxidants. KSG suppressed the inflammatory response as it decreased the genetic and protein expression as well as the levels of inflammatory cytokines. Meanwhile, KSG enhanced the mRNA expression and level of anti-inflammatory cytokine, IL-10 (interleukin-10). KSG repressed the activation of NF-κB (nuclear-factor kappa-B), besides it promoted the activation of Nrf2 signaling. Additionally, KSG markedly hindered the elevation of ASK-1 (apoptosis-signal regulating-kinase-1) and JNK (c-Jun-N-terminal kinase). Furthermore, KSG suppressed APAP-induced apoptosis as it decreased the level and expression of Bax (BCL2-associated X-protein), and caspase-3 concurrent with an enhancement of anti-apoptotic protein, Bcl2 in the liver. More thoroughly, Computational studies reveal indispensable binding affinities between KSG and Keap1 (Kelch-like ECH-associated protein-1), ASK1 (apoptosis signal-regulating kinase-1), and JNK1 (c-Jun N-terminal protein kinase-1) with distinctive tendencies for selective inhibition. Taken together, our data showed the hepatoprotective capacity of KSG against APAP-produced ALI via modulation of Nrf2/NF-κB and JNK/ASK-1/caspase-3 signaling. Henceforth, KSG could be a promising hepatoprotective candidate for ALI.

Acetaminophen Hepatotoxicity: Paradigm for Understanding Mechanisms of Drug-Induced Liver Injury

Acetaminophen (APAP) overdose is the clinically most relevant drug hepatotoxicity in western countries, and, because of translational relevance of animal models, APAP is mechanistically the most studied drug. This review covers intracellular signaling events starting with drug metabolism and the central role of mitochondrial dysfunction involving oxidant stress and peroxynitrite. Mitochondria-derived endonucleases trigger nuclear DNA fragmentation, the point of no return for cell death. In addition, adaptive mechanisms that limit cell death are discussed including autophagy, mitochondrial morphology changes, and biogenesis. Extensive evidence supports oncotic necrosis as the mode of cell death; however, a partial overlap with signaling events of apoptosis, ferroptosis, and pyroptosis is the basis for controversial discussions. Furthermore, an update on sterile inflammation in injury and repair with activation of Kupffer cells, monocyte-derived macrophages, and neutrophils is provided. Understanding these mechanisms of cell death led to discovery of N-acetylcysteine and recently fomepizole as effective antidotes against APAP toxicity.

Small-molecule inhibitors targeting apoptosis signal-regulated kinase 1

Apoptosis signal regulated kinase 1 (ASK1, also known as MAP3K5) is a member of the mitogen activated protein kinase kinase kinase (MAP3K) family. Since its first isolation from a human macrophage library in 1996, its research has been ongoing for over 25 years. A large number of reports have revealed that ASK1, as a key activator of the p38 mitogen-activated protein kinase and c-Jun N-terminal kinase (JNK) signaling cascade, responds to various stressors, and its inhibitors have important potential value in the treatment of diseases such as inflammation, cancer, and the nervous system and so on. This review summarizes the recent development in this field, including the structure and signaling pathways of ASK1, with a particular focus on the structure-activity relationships, and the hit-to-lead optimization strategies.

Nrf2 as a therapeutic target in acetaminophen hepatotoxicity: A case study with sulforaphane

Acetaminophen (APAP) overdose can cause severe liver injury and acute liver failure. The only clinically approved antidote, N-acetylcysteine (NAC), is highly effective but has a narrow therapeutic window. In the last 2 decades, activation of the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2), which regulates acute phase proteins and antioxidant defense genes, has emerged as a putative new therapeutic target against APAP hepatotoxicity. However, virtually all studies that propose Nrf2 activation as mechanism of protection used prolonged pretreatment, which is not a clinically feasible approach to treat a drug overdose. Therefore, the objective of this study was to assess if therapeutic activation of Nrf2 is a viable approach to treat liver injury after APAP overdose. We used the water-soluble Nrf2 activator sulforaphane (SFN; 5 mg/kg) in a murine model of APAP hepatotoxicity (300 mg/kg). Our results indicate that short-term treatment (≤3 h) with SFN alone did not activate Nrf2 or its target genes. However, posttreatment with SFN after APAP partially protected at 6 h likely due to more rapid activation of the Nrf2-target gene heme oxygenase-1. A direct comparison of SFN with NAC given at 1 h after APAP showed a superior protection with NAC, which was maintained at 24 h unlike with SFN. Thus, Nrf2 activators have inherent problems like the need to create a cellular stress to activate Nrf2 and delayed adaptive responses which may hamper sustained protection against APAP hepatotoxicity. Thus, compared to the more direct acting antidote NAC, Nrf2 activators are less suitable for this indication.

Effects of toxicants on endoplasmic reticulum stress and hepatic cell fate determination

Toxicant-induced injury is a significant global health issue. However, the mechanisms through which toxicants such as carbon tetrachloride, acetaminophen, dimethylformamide, cocaine, and morphine induce the death of multiple cell types and contribute to liver toxicity are highly complex. This phenomenon involves intricate signaling pathways in association with oxidative stress, inflammation, and activation of death receptors, which are closely linked to endoplasmic reticulum (ER) stress. ER stress initially triggers the unfolded protein response, which either promotes cell survival or causes cell death at later times, depending on the severity and duration of the stress. Thus, comprehending the molecular basis governing cell fate determination in the context of ER stress may provide key insights into the prevention and treatment of toxicant-induced injury. This review summarizes our current understanding of agents that trigger different forms of ER stress-mediated cell death, necroptosis, ferroptosis, pyroptosis, and apoptosis, and covers the underlying molecular basis of toxicant-induced ER stress, as well as potential target molecules.

Advances in the understanding of acetaminophen toxicity mechanisms: a clinical toxicology perspective

INTRODUCTION

Acetaminophen (paracetamol) is a commonly used analgesic and antipyretic agent, which is safe in therapeutic doses. Acetaminophen poisoning due to self-harm or repeated supratherapeutic ingestion is a common cause of acute liver injury. Acetylcysteine has been a mainstay of treatment for acetaminophen poisoning for decades and is efficacious if administered early. However, treatment failures occur if administered late, in 'massive' overdoses or in high-risk patients.

AREAS COVERED

This review provides an overview of the mechanisms of toxicity of acetaminophen poisoning (metabolic and oxidative phase) and how this relates to the assessment and treatment of the acetaminophen poisoned patient. The review focuses on how these advances offer further insight into the utility of novel biomarkers and the role of proposed adjunct treatments.

EXPERT OPINION

Advances in our understanding of acetaminophen toxicity have allowed the development of novel biomarkers and a better understanding of how adjunct treatments may prevent acetaminophen toxicity. Newly proposed adjunct treatments like fomepizole are being increasingly used without robust clinical trials. Novel biomarkers (not yet clinically available) may provide better assessment of these newly proposed adjunct treatments, particularly in clinical trials. These advances in our understanding of acetaminophen toxicity and liver injury hold promise for improved diagnosis and treatment.

Mitochondria in Acetaminophen-Induced Liver Injury and Recovery: A Concise Review

Mitochondria are critical organelles responsible for the maintenance of cellular energy homeostasis. Thus, their dysfunction can have severe consequences in cells responsible for energy-intensive metabolic function, such as hepatocytes. Extensive research over the last decades have identified compromised mitochondrial function as a central feature in the pathophysiology of liver injury induced by an acetaminophen (APAP) overdose, the most common cause of acute liver failure in the United States. While hepatocyte mitochondrial oxidative and nitrosative stress coupled with induction of the mitochondrial permeability transition are well recognized after an APAP overdose, recent studies have revealed additional details about the organelle's role in APAP pathophysiology. This concise review highlights these new advances, which establish the central role of the mitochondria in APAP pathophysiology, and places them in the context of earlier information in the literature. Adaptive alterations in mitochondrial morphology as well as the role of cellular iron in mitochondrial dysfunction and the organelle's importance in liver recovery after APAP-induced injury will be discussed.

Advances in Immunosuppressive Agents Based on Signal Pathway

Immune abnormality involves in various diseases, such as infection, allergic diseases, autoimmune diseases, as well as transplantation. Several signal pathways have been demonstrated to play a central role in the immune response, including JAK/STAT, NF-κB, PI3K/AKT-mTOR, MAPK, and Keap1/Nrf2/ARE pathway, in which multiple targets have been used to develop immunosuppressive agents. In recent years, varieties of immunosuppressive agents have been approved for clinical use, such as the JAK inhibitor tofacitinib and the mTOR inhibitor everolimus, which have shown good therapeutic effects. Additionally, many immunosuppressive agents are still in clinical trials or preclinical studies. In this review, we classified the immunosuppressive agents according to the immunopharmacological mechanisms, and summarized the phase of immunosuppressive agents.

Activation of the adenosine A2B receptor even beyond the therapeutic window of N-acetylcysteine accelerates liver recovery after an acetaminophen overdose

Acetaminophen (APAP) overdose is the most common cause of acute liver failure in the USA. The short therapeutic window of the current antidote, N-acetylcysteine (NAC) highlights the need for novel late acting therapeutics. The neuronal guidance cue netrin-1 provides delayed protection against APAP hepatotoxicity through the adenosine A2B receptor (A2BAR). The clinical relevance of this mechanism was investigated here by administration of the A2BAR agonist BAY 60-6583, after an APAP overdose (300 or 600 mg/kg) in fasted male and female C57BL/6J mice with assessment of liver injury 6 or 24 h after APAP in comparison to NAC. BAY 60-6583 treatment 1.5 h after APAP overdose (600 mg/kg) protected against liver injury at 6 h by preserving mitochondrial function despite JNK activation and its mitochondrial translocation. Gender independent protection was sustained when BAY 60-6583 was given 6 h after APAP overdose (300 mg/kg), when NAC administration did not show benefit. This protection was accompanied by enhanced infiltration of macrophages with the reparative anti-inflammatory phenotype by 24 h, accompanied by a decrease in neutrophil infiltration. Thus, our data emphasize the remarkable therapeutic utility of using an A2BAR agonist, which provides delayed protection long after the standard of care NAC ceased to be effective.

ASK1 is a novel molecular target for preventing aminoglycoside-induced hair cell death

Aminoglycoside antibiotics are lifesaving medicines, crucial for the treatment of chronic or drug resistant infections. However, aminoglycosides are toxic to the sensory hair cells in the inner ear. As a result, aminoglycoside-treated individuals can develop permanent hearing loss and vestibular impairment. There is considerable evidence that reactive oxygen species (ROS) production and the subsequent phosphorylation of c-Jun N-terminal kinase (JNK) and P38 mitogen-activated protein kinase (P38) drives apoptosis in aminoglycoside-treated hair cells. However, treatment strategies that directly inhibit ROS, JNK, or P38 are limited by the importance of these molecules for normal cellular function. Alternatively, the upstream regulator apoptosis signal-regulating kinase 1 (ASK1/MAP3K5) is a key mediator of ROS-induced JNK and P38 activation under pathologic but not homeostatic conditions. We investigated ASK1 as a mediator of drug-induced hair cell death using cochlear explants from Ask1 knockout mice, demonstrating that Ask1 deficiency attenuates neomycin-induced hair cell death. We then evaluated pharmacological inhibition of ASK1 with GS-444217 as a potential otoprotective therapy. GS-444217 significantly attenuated hair cell death in neomycin-treated explants but did not impact aminoglycoside efficacy against P. aeruginosa in the broth dilution test. Overall, we provide significant pre-clinical evidence that ASK1 inhibition represents a novel strategy for preventing aminoglycoside ototoxicity. KEY MESSAGES: • ASK1 is an upstream, redox-sensitive regulator of P38 and JNK, which are known mediators of hair cell death. • Ask1 knockout does not affect hair cell development in vivo, but significantly reduces aminoglycoside-induced hair cell death in vitro. • A small-molecule inhibitor of ASK1 attenuates neomycin-induced hair cell death, and does not impact antibiotic efficacy in vitro. • ASK1 may be a novel molecular target for preventing aminoglycoside-induced hearing loss.

Comparing N-acetylcysteine and 4-methylpyrazole as antidotes for acetaminophen overdose

Acetaminophen (APAP) overdose can cause hepatotoxicity and even liver failure. N-acetylcysteine (NAC) is still the only FDA-approved antidote against APAP overdose 40 years after its introduction. The standard oral or intravenous dosing regimen of NAC is highly effective for patients with moderate overdoses who present within 8 h of APAP ingestion. However, for late-presenting patients or after ingestion of very large overdoses, the efficacy of NAC is diminished. Thus, additional antidotes with an extended therapeutic window may be needed for these patients. Fomepizole (4-methylpyrazole), a clinically approved antidote against methanol and ethylene glycol poisoning, recently emerged as a promising candidate. In animal studies, fomepizole effectively prevented APAP-induced liver injury by inhibiting Cyp2E1 when treated early, and by inhibiting c-jun N-terminal kinase (JNK) and oxidant stress when treated after the metabolism phase. In addition, fomepizole treatment, unlike NAC, prevented APAP-induced kidney damage and promoted hepatic regeneration in mice. These mechanisms of protection (inhibition of Cyp2E1 and JNK) and an extended efficacy compared to NAC could be verified in primary human hepatocytes. Furthermore, the formation of oxidative metabolites was eliminated in healthy volunteers using the established treatment protocol for fomepizole in toxic alcohol and ethylene glycol poisoning. These mechanistic findings, together with the excellent safety profile after methanol and ethylene glycol poisoning and after an APAP overdose, suggest that fomepizole may be a promising antidote against APAP overdose that could be useful as adjunct treatment to NAC. Clinical trials to support this hypothesis are warranted.

Strategies for Improving Photodynamic Therapy Through Pharmacological Modulation of the Immediate Early Stress Response

Photodynamic therapy (PDT) is a minimally to noninvasive treatment modality that has emerged as a promising alternative to conventional cancer treatments. PDT induces hyperoxidative stress and disrupts cellular homeostasis in photosensitized cancer cells, resulting in cell death and ultimately removal of the tumor. However, various survival pathways can be activated in sublethally afflicted cancer cells following PDT. The acute stress response is one of the known survival pathways in PDT, which is activated by reactive oxygen species and signals via ASK-1 (directly) or via TNFR (indirectly). The acute stress response can activate various other survival pathways that may entail antioxidant, pro-inflammatory, angiogenic, and proteotoxic stress responses that culminate in the cancer cell's ability to cope with redox stress and oxidative damage. This review provides an overview of the immediate early stress response in the context of PDT, mechanisms of activation by PDT, and molecular intervention strategies aimed at inhibiting survival signaling and improving PDT outcome.

Emerging pharmacological treatment options for MAFLD

Metabolic dysfunction-associated fatty liver disease (MAFLD) prevalence and incidence is rising globally. It is associated with metabolic comorbidities, obesity, overweight, type 2 diabetes mellitus, and at least two metabolic risk factors, such as hypertension, hypertriglyceridemia, hypercholesterolemia, insulin resistance, and cardiovascular risk, increasing the risk of mortality. The excessive accumulation of fat comprises apoptosis, necrosis, inflammation and ballooning degeneration progressing to fibrosis, cirrhosis, and liver decompensations including hepatocellular carcinoma development. The limitation of approved drugs to prevent MAFLD progression is a paradigm. This review focuses on recent pathways and targets with evidence results in phase II/III clinical trials.

Structural Insights Support Targeting ASK1 Kinase for Therapeutic Interventions

Apoptosis signal-regulating kinase (ASK) 1, a member of the mitogen-activated protein kinase kinase kinase (MAP3K) family, modulates diverse responses to oxidative and endoplasmic reticulum (ER) stress and calcium influx. As a crucial cellular stress sensor, ASK1 activates c-Jun N-terminal kinases (JNKs) and p38 MAPKs. Their excessive and sustained activation leads to cell death, inflammation and fibrosis in various tissues and is implicated in the development of many neurological disorders, such as Alzheimer’s, Parkinson’s and Huntington disease and amyotrophic lateral sclerosis, in addition to cardiovascular diseases, diabetes and cancer. However, currently available inhibitors of JNK and p38 kinases either lack efficacy or have undesirable side effects. Therefore, targeted inhibition of their upstream activator, ASK1, stands out as a promising therapeutic strategy for treating such severe pathological conditions. This review summarizes recent structural findings on ASK1 regulation and its role in various diseases, highlighting prospects for ASK1 inhibition in the treatment of these pathologies.

Recommendations for the use of the acetaminophen hepatotoxicity model for mechanistic studies and how to avoid common pitfalls

Acetaminophen (APAP) is a widely used analgesic and antipyretic drug, which is safe at therapeutic doses but can cause severe liver injury and even liver failure after overdoses. The mouse model of APAP hepatotoxicity recapitulates closely the human pathophysiology. As a result, this clinically relevant model is frequently used to study mechanisms of drug-induced liver injury and even more so to test potential therapeutic interventions. However, the complexity of the model requires a thorough understanding of the pathophysiology to obtain valid results and mechanistic information that is translatable to the clinic. However, many studies using this model are flawed, which jeopardizes the scientific and clinical relevance. The purpose of this review is to provide a framework of the model where mechanistically sound and clinically relevant data can be obtained. The discussion provides insight into the injury mechanisms and how to study it including the critical roles of drug metabolism, mitochondrial dysfunction, necrotic cell death, autophagy and the sterile inflammatory response. In addition, the most frequently made mistakes when using this model are discussed. Thus, considering these recommendations when studying APAP hepatotoxicity will facilitate the discovery of more clinically relevant interventions.

Use of fomepizole (4-methylpyrazole) for acetaminophen poisoning: A scoping review

INTRODUCTION

Acetaminophen (paracetamol, APAP) poisoning is a prominent global cause of drug-induced liver injury. While N-acetylcysteine (NAC) is an effective antidote, it has therapeutic limitations in massive overdose or delayed presentation. The objective is to comprehensively review the literature on fomepizole as a potential adjunct antidote for acetaminophen toxicity.

METHODS

A scoping review was performed using standardized search terms from inception through July 2021.

RESULTS

Reports on fomepizole as a therapeutic adjunct for APAP toxicity span heterogeneous types of evidence. Eleven preclinical studies (in vitro and animal), fourteen case reports/series, and one human volunteer study were included. Fomepizole's action is mediated by inhibition of CYP2E1 to prevent oxidant stress generation, and inhibition of c-Jun N-terminal kinase (JNK) to decrease amplification of oxidant stress signaling to mitochondria. Studies have shown a reduction in oxidative metabolites likely by shunting metabolism away from CYP2E1 and a resultant decrease in liver injury in animals, independent of CYP2E1 interactions. Fomepizole has been linked to few adverse effects.

CONCLUSION

Based on in vitro and animal studies, and bolstered by case reports, fomepizole likely offers benefit as an adjunct antidote for APAP toxicity, however this remains to be shown in a human trial. NAC remains the standard of care antidote, but given that fomepizole is approved and generally safe, it may be considered for APAP toxicity as off-label use by experienced clinicians, in rare circumstances associated with increased risk of hepatotoxicity despite standard NAC dosing. The marginal clinical benefit of fomepizole adjunct therapy beyond NAC monotherapy remains to be clearly defined, and routine use for APAP overdose is premature based on current evidence.

Protective effect and mechanism of chitooligosaccharides on acetaminophen-induced liver injury

Currently, drug-induced liver injury caused by acetaminophen (APAP) is the second leading cause of human liver transplantation. The only clinical antidote treatment for APAP-induced liver injury is N-acetyl-L-cysteine (NAC), which has many side effects. Chitooligosaccharides (COS) are processed from naturally occurring chitin through chemical desalting and deproteinization, biological enzymatic hydrolysis and other processes. In this study, we constructed in vitro and in vivo models of APAP-induced liver injury to study COS of two molecular weights (MWs), which are COST (MW ≤ 1000 Da) and COSM (MW ≤ 3000 Da). The results showed that COST and COSM can significantly reduce the levels of serum ALT and AST and liver MDA, TNF-α, IL-1β and IL-6, and increase the levels and activity of GSH, SOD, GSH-Px and CAT. A mechanistic study found that COST and COSM can significantly reduce the expression of liver CYP2E1, Keap1, p-ASK1/ASK1, p-MKK4/MKK4, p-JNK/JNK, Caspase-3 and Bax and increase the expression of Nrf2, HO-1, eNOS, SOD and Bcl-XL. COST and COSM can inhibit toxic APAP metabolism, inhibit oxidative damage and the apoptosis pathway, increase activation of the liver antioxidant pathway, and ultimately ameliorate APAP-induced liver oxidative damage.

Oxidant Stress and Acetaminophen Hepatotoxicity: Mechanism-Based Drug Development

Significance: Acetaminophen (APAP) is one of the quantitively most consumed drugs worldwide. Although safe at therapeutic doses, intentional or unintentional overdosing occurs frequently causing severe liver injury and even liver failure. In the United States, 50% of all acute liver failure cases are caused by APAP overdose. However, only one antidote with a limited therapeutic window, N-acetylcysteine, is clinically approved. Thus, more effective therapeutic interventions are urgently needed. Recent Advances: Although APAP hepatotoxicity has been extensively studied for almost 50 years, particular progress has been made recently in two areas. First, there is now a detailed understanding of involvement of oxidative and nitrosative stress in the pathophysiology, with identification of the reactive species involved, their initial generation in mitochondria, amplification through the c-Jun N-terminal kinase pathway, and the mechanisms of cell death. Second, it was demonstrated in human hepatocytes and through biomarkers in vivo that the mechanisms of liver injury in animals accurately reflect the human pathophysiology, which allows the translation of therapeutic targets identified in animals to patients. Critical Issues: For progress, solid understanding of the pathophysiology of APAP hepatotoxicity and of a drug's targets is needed to identify promising new therapeutic intervention strategies and drugs, which may be applied to humans. Future Directions: In addition to further refine the mechanistic understanding of APAP hepatotoxicity and identify additional drugs with complementary mechanisms of action to prevent cell death, more insight into the mechanisms of regeneration and developing of drugs, which promote recovery, remains a future challenge. Antioxid. Redox Signal. 35, 718-733.

Protein Targets of Acetaminophen Covalent Binding in Rat and Mouse Liver Studied by LC-MS/MS

Acetaminophen (APAP) is a mild analgesic and antipyretic used commonly worldwide. Although considered a safe and effective over-the-counter medication, it is also the leading cause of drug-induced acute liver failure. Its hepatotoxicity has been linked to the covalent binding of its reactive metabolite, N-acetyl p-benzoquinone imine (NAPQI), to proteins. The aim of this study was to identify APAP-protein targets in both rat and mouse liver, and to compare the results from both species, using bottom-up proteomics with data-dependent high resolution mass spectrometry and targeted multiple reaction monitoring (MRM) experiments. Livers from rats and mice, treated with APAP, were homogenized and digested by trypsin. Digests were then fractionated by mixed-mode solid-phase extraction prior to liquid chromatography-tandem mass spectrometry (LC-MS/MS). Targeted LC-MRM assays were optimized based on high-resolution MS/MS data from information-dependent acquisition (IDA) using control liver homogenates treated with a custom alkylating reagent yielding an isomeric modification to APAP on cysteine residues, to build a modified peptide database. A list of putative in vivo targets of APAP were screened from data-dependent high-resolution MS/MS analyses of liver digests, previous in vitro studies, as well as selected proteins from the target protein database (TPDB), an online resource compiling previous reports of APAP targets. Multiple protein targets in each species were found, while confirming modification sites. Several proteins were modified in both species, including ATP-citrate synthase, betaine-homocysteine S-methyltransferase 1, cytochrome P450 2C6/29, mitochondrial glutamine amidotransferase-like protein/ES1 protein homolog, glutamine synthetase, microsomal glutathione S-transferase 1, mitochondrial-processing peptidase, methanethiol oxidase, protein/nucleic acid deglycase DJ-1, triosephosphate isomerase and thioredoxin. The targeted method afforded better reproducibility for analysing these low-abundant modified peptides in highly complex samples compared to traditional data-dependent experiments.

Redefining IL11 as a regeneration-limiting hepatotoxin and therapeutic target in acetaminophen-induced liver injury

Acetaminophen (N-acetyl-p-aminophenol; APAP) toxicity is a common cause of liver damage. In the mouse model of APAP-induced liver injury (AILI), interleukin 11 (IL11) is highly up-regulated and administration of recombinant human IL11 (rhIL11) has been shown to be protective. Here, we demonstrate that the beneficial effect of rhIL11 in the mouse model of AILI is due to its inhibition of endogenous mouse IL11 activity. Our results show that species-matched IL11 behaves like a hepatotoxin. IL11 secreted from APAP-damaged human and mouse hepatocytes triggered an autocrine loop of NADPH oxidase 4 (NOX4)-dependent cell death, which occurred downstream of APAP-initiated mitochondrial dysfunction. Hepatocyte-specific deletion of Il11 receptor subunit alpha chain 1 (Il11ra1) in adult mice protected against AILI despite normal APAP metabolism and glutathione (GSH) depletion. Mice with germline deletion of Il11 were also protected from AILI, and deletion of Il1ra1 or Il11 was associated with reduced c-Jun N-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK) activation and quickly restored GSH concentrations. Administration of a neutralizing IL11RA antibody reduced AILI in mice across genetic backgrounds and promoted survival when administered up to 10 hours after APAP. Inhibition of IL11 signaling was associated with the up-regulation of markers of liver regenerations: cyclins and proliferating cell nuclear antigen (PCNA) as well as with phosphorylation of retinoblastoma protein (RB) 24 hours after AILI. Our data suggest that species-matched IL11 is a hepatotoxin and that IL11 signaling might be an effective therapeutic target for APAP-induced liver damage.

Histological, immunohistochemical, and molecular investigation on the hepatotoxic effect of potassium dichromate and the ameliorating role of Persea americana mill pulp extract

The current study has been designed to assess the role of Persea americana (P. americana) pulp extract on potassium dichromate-induced hepatotoxicity in rats. P. americana pulp extract administration improved the hepatic vascular congestion, blood extravasation, inflammatory cellular infiltration, Kupffer cell hyperplasia, and nuclear changes. It also significantly ameliorated hepatic interstitial and peri-portal fibrosis and caused retrieval of the PAS-positive reaction in the liver parenchyma and around the central vein with restoration of the glycogen granules. P. americana also significantly attenuated the immunohistochemical expression of NF-kβ p65 and its downstream inflammatory cytokines IL6 and TNFα in the liver parenchyma. The antioxidant effect of P. americana was evidenced by significant modulation of the three major components of the thioredoxin (Trx) antioxidant system, the Trx, the thioredoxin reductase (TrxR), and the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase along with significant increase in the level of superoxide dismutase and glutathione, and decrease in the lipid peroxidation product malondialdehyde. P. americana pulp extract also caused significant elevation of hepatic protein phosphatase 5 with subsequent down-regulation of Apoptosis signal-regulating kinase1 (ASK1) and its downstream signaling targets MAPK kinase 4 (MKK4), p38 mitogen-activated protein kinases (p38-MAPKs), the c-JUN N-terminal kinase (JNK), and the extracellular signal-regulated kinase 1/2 (ERK 1/2). Also, In conclusion, P. americana pulp extract has anti-oxidative and anti-inflammatory effects against potassium dichromate-induced hepatotoxicity.

Mitochondrial protein adduct and superoxide generation are prerequisites for early activation of c-jun N-terminal kinase within the cytosol after an acetaminophen overdose in mice

Acetaminophen (APAP) overdose is the most common cause of acute liver failure in the United States and formation of APAP-protein adducts, mitochondrial oxidant stress and activation of the mitogen activated protein (MAP) kinase c-jun N-terminal kinase (JNK) are critical for APAP-induced cell death. However, direct evidence linking these mechanistic features are lacking and were investigated by examining the early temporal course of these changes in mice after 300 mg/kg APAP. Protein adducts were detectable in the liver (0.05-0.1 nmol/mg protein) by 15 and 30 min after APAP, which increased (>500 %) selectively in mitochondria by 60 min. Cytosolic JNK activation was only evident at 60 min, and was significantly attenuated by scavenging superoxide specifically in the cytosol by TEMPO treatment. Treatment of mouse hepatocytes with APAP revealed mitochondrial superoxide generation within 15 min, accompanied by hydrogen peroxide production without change in mitochondrial respiratory function. The oxidant stress preceded JNK activation and its mitochondrial translocation. Inhibitor studies identified the putative source of mitochondrial superoxide as complex III, which released superoxide towards the intermembrane space after APAP resulting in activation of JNK in the cytosol. Our studies provide direct evidence of mechanisms involved in mitochondrial superoxide generation after NAPQI-adduct formation and its activation of the MAP kinase cascade in the cytosol, which are critical features of APAP hepatotoxicity.

Selonsertib, a potential drug for liver failure therapy by rescuing the mitochondrial dysfunction of macrophage via ASK1-JNK-DRP1 pathway

Background

Acute liver failure (ALF) is associated with a high mortality rate, and there are still no effective treatments except liver transplantation and artificial liver therapies. This study aimed to determine the effects, therapeutic window and mechanisms of selonsertib, a selective inhibitor of ASK1, for ALF therapy.

Results

Lipopolysaccharide and d-galactosamine (LPS/GalN) were used to simulate ALF. We found that selonsertib pretreatment significantly ameliorated ALF, as determined by reduced hepatic necrosis and serum alanine aminotransferase, aspartate aminotransferase and inflammatory cytokine levels. However, selonsertib is only effective early after LPS/GalN administration, and the limited therapeutic window is related to the activation and mitochondrial translocation of JNK and DRP1. Further experiments revealed that selonsertib could alleviate LPS-induced mitochondrial damage in macrophages by evaluating the mitochondrial membrane potential and mitochondrial permeability transition pore opening in macrophages. Selonsertib also suppressed the release of inflammatory cytokines from macrophages by reducing DRP1-mediated mitochondrial dysfunction, which was confirmed by using mdivi, a specific DRP1 inhibitor.

Conclusions

Selonsertib protected against LPS/GalN-induced ALF by attenuating JNK-mediated DRP1 mitochondrial translocation and then rescuing mitochondrial damage in macrophages and may have therapeutic potential for early ALF patients.

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.

Toxicological Property of Acetaminophen: The Dark Side of a Safe Antipyretic/Analgesic Drug?

Acetaminophen (paracetamol, N-acetyl-p-aminophenol; APAP) is the most popular analgesic/antipyretic agent in the world. APAP has been regarded as a safer drug compared with non-steroidal anti-inflammatory drugs (NSAIDs) particularly in terms of lower risks of renal dysfunction, gastrointestinal injury, and asthma/bronchospasm induction, even in high-risk patients such as the elderly, children, and pregnant women. On the other hand, the recent increasing use of APAP has raised concerns about its toxicity. In this article, we review recent pharmacological and toxicological findings about APAP from basic, clinical, and epidemiological studies, including spontaneous drug adverse events reporting system, especially focusing on drug-induced asthma and pre-and post-natal closure of ductus arteriosus. Hepatotoxicity is the greatest fault of APAP and the most frequent cause of drug-induced acute liver failure in Western countries. However, its precise mechanism remains unclear and no effective cure beyond N-acetylcysteine has been developed. Recent animal and cellular studies have demonstrated that some cellular events, such as c-jun N-terminal kinase (JNK) pathway activation, endoplasmic reticulum (ER) stress, and mitochondrial oxidative stress may play important roles in the development of hepatitis. Herein, the molecular mechanisms of APAP hepatotoxicity are summarized. We also discuss the not-so-familiar "dark side" of APAP as an otherwise safe analgesic/antipyretic drug.

Acetaminophen Test Battery (ATB): A Comprehensive Method to Study Acetaminophen-Induced Acute Liver Injury

Acetaminophen (APAP) overdose is the major cause of acute liver failure (ALF) in the Western world. Extensive research is ongoing to identify the mechanisms of APAP-induced ALF. APAP-induced acute liver injury is also one of the most commonly studied drug-induced liver injury models in the field of hepatotoxicity. APAP toxicity is triphasic and includes three mechanistically interlinked but temporally distinct phases of initiation, progression, and recovery/regeneration. Despite how commonly it is studied, the methods to study APAP toxicity differ significantly, often leading to confusing and contradictory data. There are number of reviews on mechanisms of APAP toxicity, but a detailed mechanism-based comprehensive method and list of assays that covers all phases of APAP hepatotoxicity are missing. The goal of this review is to provide a standard protocol and guidelines to study APAP toxicity in mice including a test battery that can help investigators to comprehensively analyze APAP toxicity in the specific context of their hypothesis. Further, we will identify the major roadblocks and common technical problems that can significantly affect the results. This acetaminophen test battery (ATB) will be an excellent guide for scientists studying this most common and clinically relevant drug-induced liver injury and will also be helpful as a roadmap for hypothesis development to study novel mechanisms.

Novel strategies for the treatment of acetaminophen hepatotoxicity

INTRODUCTION

Acetaminophen (APAP) hepatotoxicity is the leading cause of acute liver failure in the western world. Despite extensive investigations into the mechanisms of cell death, only a single antidote, N-acetylcysteine, is in clinical use. However, there have recently been more efforts made to translate mechanistic insight into identification of therapeutic targets and potential new drugs for this indication.

AREAS COVERED

After a short review of the key events in the pathophysiology of APAP-induced liver injury and recovery, the pros and cons of targeting individual steps in the pathophysiology as therapeutic targets are discussed. While the re-purposed drug fomepizole (4-methylpyrazole) and the new entity calmangafodipir are most advanced based on the understanding of their mechanism of action, several herbal medicine extracts and their individual components are also considered.

EXPERT OPINION

Fomepizole (4-methylpyrazole) is safe and has shown efficacy in preclinical models, human hepatocytes and in volunteers against APAP overdose. The safety of calmangafodipir in APAP overdose patients was shown but it lacks solid preclinical efficacy studies. Both drugs require a controlled phase III trial to achieve regulatory approval. All studies of herbal medicine extracts and components suffer from poor experimental design, which questions their clinical utility at this point.

Review: Usnic acid-induced hepatotoxicity and cell death

Increasing prevalence of herbal and dietary supplement-induced hepatotoxicity has been reported worldwide. Usnic acid (UA) is a well-known hepatotoxin derived from lichens. Since 2000, more than 20 incident reports have been received by the US Food and Drug Administration after intake of UA containing dietary supplement resulting in severe complications. Scientists and clinicians have been studying the cause, prevention and treatment of UA-induced hepatotoxicity. It is now known that UA decouples oxidative phosphorylation, induces adenosine triphosphate (ATP) depletion, decreases glutathione (GSH), and induces oxidative stress markedly leading to lipid peroxidation and organelle stress. In addition, experimental rat liver tissues have shown massive vacuolization associated with cellular swellings. Additionally, various signaling pathways, such as c-JNK N-terminal kinase (JNK), store-operated calcium entry, nuclear erythroid 2-related factor 2 (Nrf2), and protein kinase B/mammalian target of rapamycin (Akt/mTOR) pathways are stimulated by UA causing beneficial or harmful effects. Nevertheless, there are controversial issues, such as UA-induced inflammatory or anti-inflammatory responses, cytochrome P450 detoxifying UA into non-toxic or transforming UA into reactive metabolites, and unknown mechanism of the formation of vacuolization and membrane pore. This article focused on the previous and latest comprehensive putative mechanistic findings of UA-induced hepatotoxicity and cell death. New insights on controversial issues and future perspectives are also discussed and summarized.

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.

Antihepatic Fibrosis Drugs in Clinical Trials

Liver fibrosis is not an independent disease. It refers to the abnormal proliferation of connective tissues in the liver caused by various pathogenic factors. Thus far, liver fibrosis has been considered to be associated with a set of factors, such as viral infection, alcohol abuse, non-alcoholic fatty liver disease, and autoimmune hepatitis, as well as genetic diseases. To date, clinical therapeutics for liver fibrosis still face challenges, as elimination of potential causes and conventional antifibrotic drugs cannot alleviate fibrosis in most patients. Recently, potential therapeutic targets of liver fibrosis, such as metabolism, inflammation, cell death and the extracellular matrix, have been explored through basic and clinical research. Therefore, it is extremely urgent to review the antihepatic fibrosis therapeutics for treatment of liver fibrosis in current clinical trials.

Delayed Treatment With 4-Methylpyrazole Protects Against Acetaminophen Hepatotoxicity in Mice by Inhibition of c-Jun n-Terminal Kinase

Acetaminophen (APAP) overdose is the most common cause of hepatotoxicity and acute liver failure in the United States and many western countries. However, the only clinically approved antidote, N-acetylcysteine, has a limited therapeutic window. 4-Methylpyrazole (4MP) is an antidote for methanol and ethylene glycol poisoning, and we have recently shown that cotreatment of 4MP with APAP effectively prevents toxicity by inhibiting Cyp2E1. To evaluate if 4MP can be used therapeutically, C57BL/6J mice were treated with 300 mg/kg APAP followed by 50 mg/kg 4MP 90 min later (after the metabolism phase). In these experiments, 4MP significantly attenuated liver injury at 3, 6, and 24 h after APAP as shown by 80%-90% reduction in plasma alanine aminotransferase activities and reduced areas of necrosis. 4MP prevented c-Jun c-Jun N-terminal kinase (JNK) activation and its mitochondrial translocation, and reduced mitochondrial oxidant stress and nuclear DNA fragmentation. 4MP also prevented JNK activation in other liver injury models. Molecular docking experiments showed that 4MP can bind to the ATP binding site of JNK. These data suggest that treatment with 4MP after the metabolism phase effectively prevents APAP-induced liver injury in the clinically relevant mouse model in vivo mainly through the inhibition of JNK activation. 4MP, a drug approved for human use, is as effective as N-acetylcysteine or can be even more effective in cases of severe overdoses with prolonged metabolism (600 mg/kg). 4MP acts on alternative therapeutic targets and thus may be a novel approach to treatment of APAP overdose in patients that complements N-acetylcysteine.

Selonsertib for patients with bridging fibrosis or compensated cirrhosis due to NASH: Results from randomized phase III STELLAR trials

BACKGROUND & AIMS

Apoptosis signal-regulating kinase 1 (ASK1) plays a key role in hepatocyte injury, inflammation, and fibrosis in non-alcoholic steatohepatitis (NASH). We evaluated the safety and antifibrotic effect of selonsertib, a selective inhibitor of ASK1, in patients with advanced fibrosis due to NASH.

METHODS

We conducted 2 randomized, double-blind, placebo-controlled, phase III trials of selonsertib in patients with NASH and bridging fibrosis (F3, STELLAR-3) or compensated cirrhosis (F4, STELLAR-4). Patients were randomized 2:2:1 to receive selonsertib 18 mg, selonsertib 6 mg, or placebo once daily for 48 weeks. Liver biopsies were performed at screening and week 48 and non-invasive tests of fibrosis (NITs) were evaluated. The primary efficacy endpoint was the proportion of patients with ≥1-stage improvement in fibrosis without worsening of NASH at week 48. Additional endpoints included changes in NITs, progression to cirrhosis (in STELLAR-3), and liver-related clinical events.

RESULTS

Neither trial met the primary efficacy endpoint. In STELLAR-3, fibrosis improvement without worsening of NASH was observed in 10% (31/322, p = 0.49 vs. placebo), 12% (39/321, p = 0.93 vs. placebo), and 13% (21/159) of patients in the selonsertib 18 mg, selonsertib 6 mg, and placebo groups, respectively. In STELLAR-4, the primary endpoint was achieved in 14% (51/354; p = 0.56), 13% (45/351; p = 0.93), and 13% (22/172) of patients, respectively. Although selonsertib led to dose-dependent reductions in hepatic phospho-p38 expression indicative of pharmacodynamic activity, it had no significant effect on liver biochemistry, NITs, progression to cirrhosis, or adjudicated clinical events. The rates and types of adverse events were similar among selonsertib and placebo groups.

CONCLUSIONS

Forty-eight weeks of selonsertib monotherapy had no antifibrotic effect in patients with bridging fibrosis or compensated cirrhosis due to NASH.

LAY SUMMARY

Patients with non-alcoholic steatohepatitis (NASH) can develop scarring of the liver (fibrosis), including cirrhosis, which increases the risks of liver failure and liver cancer. We tested whether 48 weeks of treatment with selonsertib reduced fibrosis in patients with NASH and advanced liver scarring. We did not find that selonsertib reduced fibrosis in these patients.

TRIAL REGISTRATION DETAILS

Clinicaltrials.gov numbers NCT03053050 and NCT03053063.

Autophagy: a promising process for the treatment of acetaminophen-induced liver injury

Toxicity from drugs has become an important cause of acute liver failure. Acetaminophen, a commonly used analgesic, can cause severe acute liver injury that can worsen into acute liver failure. Autophagy, a protective cell programme, has been reported to have protective effects in a variety of diseases such as cancer, immune diseases, neurodegenerative diseases, and inflammatory diseases. In this review, we describe how an excess of acetaminophen causes liver injury step by step, from the formation of the initial protein adduct to the final hepatocyte necrosis, as well as the induction of autophagy and its beneficial effects on diseases. Emphasis is placed on the potential effect of autophagy on improving the damage of acetaminophen to hepatocytes. Finally, we are committed to providing insights into the treatment of acute liver failure through the mechanism of acetaminophen induced liver injury, the mechanism of autophagy, and the link between autophagy and liver injury.

Possible Pathways of Hepatotoxicity Caused by Chemical Agents

BACKGROUND

Liver injury induced by drugs has become a primary reason for acute liver disease and therefore posed a potential regulatory and clinical challenge over the past few decades and has gained much attention. It also remains the most common cause of failure of drugs during clinical trials. In 50% of all acute liver failure cases, drug-induced hepatoxicity is the primary factor and 5% of all hospital admissions.

METHODS

The various hepatotoxins used to induce hepatotoxicity in experimental animals include paracetamol, CCl4, isoniazid, thioacetamide, erythromycin, diclofenac, alcohol, etc. Among the various models used to induce hepatotoxicity in rats, every hepatotoxin causes toxicity by different mechanisms.

RESULTS

The drug-induced hepatotoxicity caused by paracetamol accounts for 39% of the cases and 13% hepatotoxicity is triggered by other hepatotoxic inducing agents.

CONCLUSION

Research carried out and the published papers revealed that hepatotoxins such as paracetamol and carbon- tetrachloride are widely used for experimental induction of hepatotoxicity in rats.

Discovery and development of ASK1 inhibitors

Aberrant activation of mitogen-activated protein kinases (MAPKs) like c-Jun N-terminal kinase (JNK) and p38 is an event involved in the pathophysiology of numerous human diseases. The apoptosis signal-regulating kinase 1 (ASK1) is an upstream target that gets activated only under pathological conditions and as such is a promising target for therapeutic intervention. In the first part of this review the molecular mechanisms leading to ASK1 activation and regulation will be described as well as the evidences supporting a pathogenic role for ASK1 in human disease. In the second part, an update on drug discovery efforts towards the discovery and development of ASK1-targeting therapies will be provided.

Mitochondrial depolarization and repolarization in the early stages of acetaminophen hepatotoxicity in mice

Mitochondrial injury and depolarization are primary events in acetaminophen hepatotoxicity. Previous studies have shown that restoration of mitochondrial function in surviving hepatocytes, which is critical to recovery, is at least partially accomplished via biogenesis of new mitochondria. However, other studies indicate that mitochondria also have the potential to spontaneously repolarize. Although repolarization was previously observed only at a sub-hepatotoxic dose of acetaminophen, we postulated that mitochondrial repolarization in hepatocytes outside the centrilobular regions of necrosis might contribute to recovery of mitochondrial function following acetaminophen-induced injury. Our studies utilized longitudinal intravital microscopy of millimeter-scale regions of the mouse liver to characterize the spatio-temporal relationship between mitochondrial polarization and necrosis early in acetaminophen-induced liver injury. Treatment of male C57BL/6J mice with a single intraperitoneal 250 mg/kg dose of acetaminophen resulted in hepatotoxicity that was apparent histologically within 2 h of treatment, leading to 20 and 60-fold increases in serum aspartate aminotransferase and alanine aminotransferase, respectively, within 6 h. Intravital microscopy of the livers of mice injected with rhodamine123, TexasRed-dextran, propidium iodide and Hoechst 33342 detected centrilobular foci of necrosis within extended regions of mitochondrial depolarization within 2 h of acetaminophen treatment. Although regions of necrosis were more apparent 6 h after acetaminophen treatment, the vast majority of hepatocytes with depolarized mitochondria did not progress to necrosis, but rather recovered mitochondrial polarization within 6 h. Recovery of mitochondrial function following acetaminophen hepatotoxicity thus involves not only biogenesis of new mitochondria, but also repolarization of existing mitochondria. These studies also revealed a spatial distribution of necrosis and mitochondrial depolarization whose single-cell granularity is inconsistent with the hypothesis that communication between neighboring cells plays an important role in the propagation of necrosis during the early stages of APAP hepatotoxicity. Small islands of healthy, intact cells were frequently found surrounded by necrotic cells, and small islands of necrotic cells were frequently found surrounded by healthy, intact cells. Time-series studies demonstrated that these "islands", consisting in some cases of single cells, are persistent; over a period of hours, injury does not spread from individual necrotic cells to their neighbors.

Emerging targets and potential therapeutic agents in non-alcoholic fatty liver disease treatment

Nonalcoholic Fatty Liver Disease (NAFLD) is the most common chronic liver disease in the world, which is characterized by liver fat accumulation unrelated to excessive drinking. Indeed, it attracts growing attention and becomes a global health problem. Due to the complexity of the NAFLD pathogenic mechanism, no related drugs were approved by Food and Drug Administration (FDA) till now. However, it is encouraging that a series of candidate drugs have entered the clinical trial stage with expectation to treat NAFLD. In this review, we summarized the main pathways and pathogenic mechanisms of NAFLD, as well as introduced the main potential therapeutic targets and the corresponding compounds involved in metabolism, inflammation and fibrosis. Furthermore, we also discuss the progress of these compounds, such as drug design and optimization, the choice of pharmacological properties and druglikeness, and the analysis of structure-activity relationship. This review offers a medium on future drug design and development, to be beneficial to relevant studies.

THE ROLE OF OXIDANT STRESS IN ACETAMINOPHE-INDUCED LIVER INJURY

Acetaminophen is a widely used analgesic and antipyretic, which can cause liver injury after an overdose. Although a controversial topic for some time, solid evidence for a critical role of oxidative and nitrosative stress has emerged during the last two decades. This review will discuss the cellular sources, amplification mechanisms and the consequences of the excessive formation of reactive oxygen and nitrogen species in the clinically relevant mouse model of acetaminophen hepatotoxicity. This new mechanistic insight contributes to the better understanding of the mechanism of action of N-acetylcysteine, the only clinically approved antidote. In addition, it provides the rationale for the development of new antidotes that target the formation or metabolism of mitochondrial superoxide.

ASK1 inhibition: a therapeutic strategy with multi-system benefits

p38 mitogen-activated protein kinases (P38α and β) and c-Jun N-terminal kinases (JNK1, 2, and 3) are key mediators of the cellular stress response. However, prolonged P38 and JNK signalling is associated with damaging inflammatory responses, reactive oxygen species-induced cell death, and fibrosis in multiple tissues, such as the kidney, liver, central nervous system, and cardiopulmonary systems. These responses are associated with many human diseases, including arthritis, dementia, and multiple organ dysfunctions. Attempts to prevent P38- and JNK-mediated disease using small molecule inhibitors of P38 or JNK have generally been unsuccessful. However, apoptosis signal-regulating kinase 1 (ASK1), an upstream regulator of P38 and JNK, has emerged as an alternative drug target for limiting P38- and JNK-mediated disease. Within this review, we compile the evidence that ASK1 mediates damaging cellular responses via prolonged P38 or JNK activation. We discuss the potential benefits of ASK1 inhibition as a therapeutic and summarise the studies that have tested the effects of ASK1 inhibition in cell and animal disease models, in addition to human clinical trials for a variety of disorders.

Acetaminophen-Induced Liver Damage in Hepatic Steatosis

One of the most used painkillers is acetaminophen (APAP), which is safe at the right dose. However, several studies have described populations susceptible to APAP-induced liver damage, mainly in livers with steatosis. Thus, clinicians should consider the presence of obesity and other chronic liver diseases like nonalcoholic fatty liver disease (NAFLD) when indicating treatment with APAP. Liver damage from this drug is generated through its metabolite N-acetyl-p-benzoquinone imine, which is detoxified with glutathione (GSH). Prior depletion of GSH in steatotic hepatocytes plays a key role in APAP-induced hepatotoxicity in people with obesity and NAFLD. The knowledge about the damage to the liver or APAP in susceptible people like the obese and those with NAFLD is of great relevance for the sanitary sector because it would imply strategies of different therapeutic approach in such patients. This paper reviews the role of APAP in liver damage in the presence of obesity, NAFLD, and nonalcoholic steatohepatitis.

Mitochondrial Damage and Biogenesis in Acetaminophen-induced Liver Injury

Liver injury and acute liver failure caused by acetaminophen (APAP) overdose is the clinically most important drug toxicity in western countries. Mechanistic investigations have revealed a central role of mitochondria in the pathophysiology. Excess formation of the reactive metabolite N-acetyl-p-benzoquinone imine (NAPQI) after an overdose leads to hepatic glutathione depletion, mitochondrial protein adducts formation and an initial oxidant stress, which triggers the activation of mitogen activated protein (MAP) kinase cascade ultimately leading to c-jun N-terminal kinase (JNK) phosphorylation. Phospho-JNK translocates to the mitochondria and amplifies the oxidative and nitrosative stress eventually causing the mitochondrial membrane permeability transition pore opening and cessation of ATP synthesis. In addition, mitochondrial matrix swelling ruptures the outer membrane and releases endonucleases, which cause nuclear DNA fragmentation. Together, the nuclear DNA damage and the extensive mitochondrial dysfunction result in necrotic cell death. However, the pro-cell death signaling events are counteracted by adaptive responses such as autophagy and mitochondrial biogenesis. The improved mechanistic insight into the pathophysiology leads to better understanding of the mechanisms of action of the existing antidote N-acetylcysteine and justifies the clinical testing of novel therapeutics such as 4-methylpyrazole and calmangafodipir.

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.

Acetaminophen Hepatotoxicity

Acetaminophen (APAP) is one of the most popular and safe pain medications worldwide. However, due to its wide availability, it is frequently implicated in intentional or unintentional overdoses where it can cause severe liver injury and even acute liver failure (ALF). In fact, APAP toxicity is responsible for 46% of all ALF cases in the United States. Early mechanistic studies in mice demonstrated the formation of a reactive metabolite, which is responsible for hepatic glutathione depletion and initiation of the toxicity. This insight led to the rapid introduction of N-acetylcysteine as a clinical antidote. However, more recently, substantial progress was made in further elucidating the detailed mechanisms of APAP-induced cell death. Mitochondrial protein adducts trigger a mitochondrial oxidant stress, which requires amplification through a MAPK cascade that ultimately results in activation of c-jun N-terminal kinase (JNK) in the cytosol and translocation of phospho-JNK to the mitochondria. The enhanced oxidant stress is responsible for the membrane permeability transition pore opening and the membrane potential breakdown. The ensuing matrix swelling causes the release of intermembrane proteins such as endonuclease G, which translocate to the nucleus and induce DNA fragmentation. These pathophysiological signaling mechanisms can be additionally modulated by removing damaged mitochondria by autophagy and replacing them by mitochondrial biogenesis. Importantly, most of the mechanisms have been confirmed in human hepatocytes and indirectly through biomarkers in plasma of APAP overdose patients. The extensive necrosis caused by APAP overdose leads to a sterile inflammatory response. Although recruitment of inflammatory cells is necessary for removal of cell debris in preparation for regeneration, these cells have the potential to aggravate the injury. This review touches on the newest insight into the intracellular mechanisms of APAP-induced cells death and the resulting inflammatory response. Furthermore, it discusses the translation of these findings to humans and the emergence of new therapeutic interventions.

Acetaminophen hepatotoxicity: A mitochondrial perspective

Acetaminophen (APAP) is a highly effective analgesic, which is safe at therapeutic doses. However, an overdose can cause hepatotoxicity and even liver failure. APAP toxicity is currently the most common cause of acute liver failure in the United States. Decades of research on mechanisms of liver injury have established the role of mitochondria as central players in APAP-induced hepatocyte necrosis and this chapter examines the various facets of the organelle's involvement in the process of injury as well as in resolution of damage. The injury process is initiated by formation of a reactive metabolite, which binds to sulfhydryl groups of cellular proteins including mitochondrial proteins. This inhibits the electron transport chain and leads to formation of reactive oxygen species, which induce the activation of redox-sensitive members of the MAP kinase family ultimately causing activation of c-Jun N terminal kinase, JNK. Translocation of JNK to the mitochondria then amplifies mitochondrial dysfunction, ultimately resulting in mitochondrial permeability transition and release of mitochondrial intermembrane proteins, which trigger nuclear DNA fragmentation. Together, these events result in hepatocyte necrosis, while adaptive mechanisms such as mitophagy remove damaged mitochondria and minimize the extent of the injury. This oscillation between recovery and necrosis is predominant in cells at the edge of the necrotic area in the liver, where induction of mitochondrial biogenesis is important for liver regeneration. All these aspects of mitochondria in APAP hepatotoxicity, as well as their relevance to humans with APAP overdose and development of therapeutic approaches will be examined in detail in this chapter.