Plasma membrane cytochromes P450 as neoantigens and autoimmune targets in drug-induced hepatitis

Long-term sequelae of drug-induced liver injury

Drug-induced liver injury (DILI) has a very variable clinical and biochemical phenotype and differs widely in severity, from mild injury to life-threatening liver failure. Chronic injury has also been reported to occur at a variable frequency, ranging from 3.4% to 39%, 6-12 months after discontinuing the implicated agent. This wide range is probably related to various definitions of chronic liver injury and variable selection of patients. The long-term sequalae of this chronic injury in terms of morbidity and mortality are unclear, although rare vanishing bile duct syndrome is associated with an unfavourable prognosis, with increased risk of chronic liver failure and need for liver transplantation. Other forms of long-term sequalae associated with DILI are progressive fibrosis, autoimmune-like hepatitis, secondary sclerosing cholangitis, sinusoidal obstruction syndrome and, as a common final stage, the development of cirrhosis, portal hypertension and its complications. Immune checkpoint inhibitors, which can cause an autoimmune-like phenotype have also recently been shown to cause sclerosing cholangitis with cytotoxic T CD8+ cell infiltration in biliary tracts. DILI has been shown to have a significant impact on health-related quality of life but very little is known about its psychological consequences in the long-term. Further investigations with structured long-term follow-up and periodic quality of life surveys are needed to assess the impact of DILI on psychological outcomes, particularly in those with chronic sequelae.

Cytochrome P450 endoplasmic reticulum-associated degradation (ERAD): therapeutic and pathophysiological implications

The hepatic endoplasmic reticulum (ER)-anchored cytochromes P450 (P450s) are mixed-function oxidases engaged in the biotransformation of physiologically relevant endobiotics as well as of myriad xenobiotics of therapeutic and environmental relevance. P450 ER-content and hence function is regulated by their coordinated hemoprotein syntheses and proteolytic turnover. Such P450 proteolytic turnover occurs through a process known as ER-associated degradation (ERAD) that involves ubiquitin-dependent proteasomal degradation (UPD) and/or autophagic-lysosomal degradation (ALD). Herein, on the basis of available literature reports and our own recent findings of in vitro as well as in vivo experimental studies, we discuss the therapeutic and pathophysiological implications of altered P450 ERAD and its plausible clinical relevance. We specifically (i) describe the P450 ERAD-machinery and how it may be repurposed for the generation of antigenic P450 peptides involved in P450 autoantibody pathogenesis in drug-induced acute hypersensitivity reactions and liver injury, or viral hepatitis; (ii) discuss the relevance of accelerated or disrupted P450-ERAD to the pharmacological and/or toxicological effects of clinically relevant P450 drug substrates; and (iii) detail the pathophysiological consequences of disrupted P450 ERAD, contributing to non-alcoholic fatty liver disease (NAFLD)/non-alcoholic steatohepatitis (NASH) under certain synergistic cellular conditions.

Examining pathogenic concepts of autoimmune hepatitis for cues to future investigations and interventions

BACKGROUND

Multiple pathogenic mechanisms have been implicated in autoimmune hepatitis, but they have not fully explained susceptibility, triggering events, and maintenance or escalation of the disease. Furthermore, they have not identified a critical defect that can be targeted. The goals of this review are to examine the diverse pathogenic mechanisms that have been considered in autoimmune hepatitis, indicate investigational opportunities to validate their contribution, and suggest interventions that might evolve to modify their impact. English abstracts were identified in PubMed by multiple search terms. Full length articles were selected for review, and secondary and tertiary bibliographies were developed. Genetic and epigenetic factors can affect susceptibility by influencing the expression of immune regulatory genes. Thymic dysfunction, possibly related to deficient production of programmed cell death protein-1, can allow autoreactive T cells to escape deletion, and alterations in the intestinal microbiome may help overcome immune tolerance and affect gender bias. Environmental factors may trigger the disease or induce epigenetic changes in gene function. Molecular mimicry, epitope spread, bystander activation, neo-antigen production, lymphocytic polyspecificity, and disturbances in immune inhibitory mechanisms may maintain or escalate the disease. Interventions that modify epigenetic effects on gene expression, alter intestinal dysbiosis, eliminate deleterious environmental factors, and target critical pathogenic mechanisms are therapeutic possibilities that might reduce risk, individualize management, and improve outcome. In conclusion, diverse pathogenic mechanisms have been implicated in autoimmune hepatitis, and they may identify a critical factor or sequence that can be validated and used to direct future management and preventive strategies.

2'-Hydroxychalcone Induced Cytotoxicity via Oxidative Stress in the Lipid-Loaded Hepg2 Cells

Licorice is a common herb used in traditional Chinese medicine, and has been widely used clinically. Physiologically, although it is relatively safe, licorice-induced hepatotoxicity in the presence of other diseases needs to be evaluated. The present study was conducted to investigate the toxicological effects of the bioactive components of licorice in HepG2 cells cultured with or without free fatty acid (FFA). The compounds, isoliquiritigenin, licorice chalcone A, bavachalcone, and 2′-hydroxy chalcone (2′-HC) inhibited cell proliferation at certain concentrations in lipid loaded cells with limited effects on the normal cells. The representative compound 2′-HC (at a concentration of ≥ 20µM) increased the oxygen consumption rate, ATP production, mitochondrial membrane potential, generation of total and mitochondrial reactive oxygen species (ROS) production, and expression of inflammatory cytokines (TNF-α, IL-6, and IL-8) and Caspase-9 protein; and reduced the expression of SOD1. In addition, we found exaggerated lipid accumulation in HepG2 cells treated with FFA. Our results suggest that 2′-HC at a concentration of ≥ 20µM might cause damage to the hepatocytes. The toxicity may be related to excess ROS production and inadequate SOD1 expression, leading to apoptosis, inflammation, and cellular dysfunctions.

Development of Autoimmune Hepatitis during Direct-acting Antiviral Therapy for Chronic Hepatitis C Virus Infection

An 81-year-old woman developed liver dysfunction after two months' treatment with direct-acting antivirals (DAAs) for chronic hepatitis C virus (HCV) infection. She was positive for serum anti-nuclear antibody, with an elevated immunoglobulin G level. A liver biopsy revealed high-grade interface hepatitis and infiltrate of lymphocytes and plasma cells. DAA-associated drug-induced autoimmune hepatitis (DI-AIH) was considered. Her liver dysfunction improved after discontinuing DAA therapy and starting prednisolone treatment. The differential diagnosis for AIH should include liver injury during DAA therapy for chronic HCV infection.

Drug metabolism and liver disease: a drug-gene-environment interaction

Despite the central role of the liver in drug metabolism, surprisingly there is lack of certainty in anticipating the extent of modification of the clearance of a given drug in a given patient. The intent of this review is to provide a conceptual framework in considering the impact of liver disease on drug disposition and reciprocally the impact of drug disposition on liver disease. It is proposed that improved understanding of the situation is gained by considering the issue as a special example of a drug-gene-environment interaction. This requires an integration of knowledge of the drug's properties, knowledge of the gene products involved in its metabolism, and knowledge of the pathophysiology of its disposition. This will enhance the level of predictability of drug disposition and toxicity for a drug of interest in an individual patient. It is our contention that advances in pharmacology, pharmacogenomics, and hepatology, together with concerted interests in the academic, regulatory, and pharmaceutical industry communities provide an ideal immediate environment to move from a qualitative reactive approach to quantitative proactive approach in individualizing patient therapy in liver disease.

An analysis of drug-induced liver injury, which showed histological findings similar to autoimmune hepatitis

BACKGROUND

Drug-induced liver injury (DILI) sometimes resembles autoimmune hepatitis (AIH) in its hepatic histology. However, there is lacking data of a comparison of the characteristics between such DILI and DILI without histological findings like AIH.

METHODS

We enrolled 62 patients with DILI who were diagnosed using the Roussel Uclaf Causality Assessment Method, and performed a liver biopsy. These patients were classified into two groups: DILI with histology like AIH (group A, n = 23) and DILI without such histology (group B, n = 39). Sixteen patients of group A could be further classified into two groups: patients with relapse of the liver injury (group C, n = 8) and without relapse (group D, n = 8), after the recovery of the DILI. We compared the clinical and histological findings between group A and B, and group C versus D.

RESULTS

Group A was characterized by an older age (p = 0.043), higher immunoglobulin G level (p = 0.017), positive antinuclear antibody status (p = 0.044), and a higher frequency of complementary alternative medicines and Chinese herbal medicines as the causative drug (p = 0.008). There were no significant differences between group C and D regarding the clinical data and liver histological findings.

CONCLUSIONS

The clinical characteristics of DILI, which showed histological findings similar to AIH, were revealed. In such patients, a liver biopsy is recommended in order to determine the appropriate treatment strategy. In DILI with histology like AIH patients, long-term follow-up is needed to perceive the relapse.

Changes in IL-2 and IL-10 during Chronic Administration of Isoniazid, Nevirapine, and Paracetamol in Rats

The aim of this study was to illustrate the initial subclinical drug-induced liver injury and the associated adaptive immune response by monitoring for the changes in plasma IL-2, IL-10, and some cytochrome P450 activity during chronic administration of nevirapine (NVP), isoniazid (INH), and paracetamol (PAR) in rats without clinical hepatotoxicity. Male Sprague-Dawley (SD) rats were divided into four groups (saline (S), NVP, INH, and PAR) of 25 animals each. The drugs were administered daily for 42 days at therapeutic doses (NVP 200 mg/kg, PAR 500 mg/kg, and INH 20 mg/kg) to the respective groups by oral gavage and five rats per group were sacrificed weekly. All the three drugs induced a subclinical liver injury in the first 2-3 weeks followed by healing, indicating adaption. The liver injury was pathologically similar and was associated with immune stimulation and increased cytochrome P450 activity. NVP- and PAR-induced liver injury lasted up to 14 days while that for INH lasted for 28 days. NVP-induced liver injury was associated with increased IL-2, CD4 count, and CYP3A2 activity, followed by increased IL-10 during the healing phase. In conclusion, the initial drug-induced subclinical liver injury, its spontaneous healing, and the associated adaptive immune response have been demonstrated.

Oral clindamycin causing acute cholestatic hepatitis without ductopenia: a brief review of idiosyncratic drug-induced liver injury and a case report

Clindamycin is a lincosamide antibiotic active against most of the anaerobes, protozoans, and Gram-positive bacteria, including community-acquired methicillin-resistant Staphylococcus aureus. Its use has increased greatly in the recent past due to wide spectrum of activity and good bioavailability in oral form. Close to 20% of the patients taking clindamycin experience diarrhea as the most common side effect. Hepatotoxicity is a rare side effect. Systemic clindamycin therapy has been linked to two forms of hepatotoxicity: transient serum aminotransferase elevation and an acute idiosyncratic liver injury that occurs 1–3 weeks after starting therapy. This article is a case report of oral clindamycin induced acute symptomatic cholestatic hepatitis and a brief review of the topic.

Transitioning from Idiopathic to Explainable Autoimmune Hepatitis

Autoimmune hepatitis lacks an identifiable cause, and its diagnosis requires the exclusion of etiologically defined diseases that resemble it. Insights into its pathogenesis are moving autoimmune hepatitis from an idiopathic to explainable disease, and the goal of this review is to describe the insights that are hastening this transition. Two types of autoimmune hepatitis are justified by serological markers, but they also have distinctive genetic associations (DRB1 and DQB1 genes) and autoantigens. DRB1 alleles are the principal susceptibility factors in white adults, and a six amino acid sequence encoded in the antigen-binding groove of class II molecules of the major histocompatibility complex can influence the selection of autoantigens. Polymorphisms, including variants of SH2B3 and CARD10 genes, may affect immune reactivity and disease severity. The cytochrome mono-oxygenase, CYP2D6, is the autoantigen associated with type 2 autoimmune hepatitis, and it shares homologies with multiple viruses that might promote self-intolerance by molecular mimicry. Chemokines, especially CXCL9 and CXCL10, orchestrate the migration of effector cells to sites of injury and are associated with disease severity. Cells of the innate and adaptive immune responses promote tissue damage, and possible deficiencies in the number and function of regulatory T cells may facilitate the injurious process. Receptor-mediated apoptosis is the principal mechanism of hepatocyte loss, and cell-mediated and antibody-dependent mechanisms of cytotoxicity also contribute. Insights that explain autoimmune hepatitis will allow triggering exogenous antigens to be characterized, risk management to be improved, prognostic indices to be refined, and site-specific therapeutic interventions to emerge.

Histopathological and biochemical assessment of d-limonene-induced liver injury in rats

The aim of the present work was to develop a biochemical, histologic and immunohistochemical study about the potential hepatotoxic effect of d-limonene - a component of volatile oils extracted from citrus plants. Blood alkaline phosphatase (ALP), aspartate aminotransferase (AST) and alanine aminotransferase (ALT) from d-limonene-treated animals were determined and compared to morphologic hepatic lesions in order to investigate the possible physiopathologic mechanisms involved in the liver toxicity, in experimental animals treated with d-limonene. Wistar rats were randomly divided into seven groups: two control groups (untreated or receiving only vehicle, tween-80); one positive control (vehicle); two experimental groups treated with d-limonene at doses of 25 mg/kg/day and 75 mg/kg/day for 45 days, and two other groups treated with the same doses for 30 days and kept under observation during 30 more days. Biochemical data showed significant reduction in ALT levels in the animals treated with 75 mg/kg of d-limonene. Histological analysis revealed some hepatocyte morphological lesions, including hydropic degeneration, microvesicular steatosis and necrosis, Kupffer cell hyperplasia and incipient fibrosis. By immunohistochemistry, influx of T (CD3+) and cytotoxic (CD8+) lymphocytes was observed in the rats treated with d-limonene at both dose levels. In conclusion, it is possible that d-limonene has been directly responsible for hepatic parenchymal and matrix damage following subchronic treatment with d-limonene.

Acute hepatitis after starting pinaverium bromide in a patient taking mirtazapine

A 56-year-old man presented with chronic abdominal pain. He had been evaluated extensively in the recent past undergoing upper gastrointestinal endoscopy, colonoscopy and CT scan of the abdomen with normal results. The provisional diagnosis of irritable bowel syndrome was performed and pinaverium bromide was started. The patient had pre-existing hypertension, a major depressive disorder and gastro-oesophageal reflux disease. He had been taking nebivolol and pantoprazole for several years and mirtazapine for the last 1 year. The patient developed nausea, vomiting and anorexia after 5 days of starting pinaverium bromide. Investigations revealed marked elevation of liver enzymes and bilirubin. He was negative for HIV, HBSAg, anti-hepatitis C virus, IgM for hepatitis A virus, hepatitis E virus, antinuclear antibody and antimitochondrial antibody. An ultrasound showed mild hepatomegaly with hypoechoic echo texture; the rest of scan was normal. Pinaverium and mirtazapine were stopped immediately. The patient was treated symptomatically and his liver profile returned to normal after 4 weeks.

Idiosyncratic drug hepatotoxicity: a 2008 update

Pharmaceutical preparations, and also herbal products and dietary supplements, are emerging contributors to severe forms of liver disease. Although acetaminophen intoxication is still the reason for many cases of drug-induced liver injury (DILI) in Western countries, the bulk of hepatic reactions to drugs are idiosyncratic. Only a small fraction of individuals exposed to a drug associated with liver injury will develop hepatotoxicity. Indeed, the rarity of this serious adverse event prevents its detection in clinical trials. The pathogenesis of idiosyncratic DILI is not well known because of a lack of reliable animal models, although it probably involves the metabolism of the drug and/or activation of the immune system. Different databases have described antibiotics, NSAIDs and anticonvulsants as the main group of drugs incriminated in DILI. Clinical presentation of DILI includes predominantly a hepatocellular type of damage, yet cholestatic and mixed types are also common; the determinants of the type of damage induced by a given drug are poorly understood. Analysis of pooled data has recently underlined the influence of older age in the cholestatic/mixed expression of liver injury, as well as the independent association of female gender, older age, aspartate aminotransferase levels with hepatocellular type of damage and high bilirubin levels with the risk of fulminant liver failure/death. In the long term (providing the patient survives the initial episode), persistent damage may occur in at least 6% of patients, with the cholestatic mixed type of damage more prone to becoming chronic, while in the hepatocellular pattern the severity is greater, with further likelihood of evolution to cirrhosis. Cardiovascular and CNS drugs are the main groups leading to chronic liver damage. The diagnosis of hepatotoxicity remains a difficult task owing to the lack of reliable markers for use in general clinical practice. Diagnostic algorithms may add consistency to clinical judgment by translating a suspicion into a quantitative score. Currently, the Council for International Organizations of Medical Sciences/Roussel Uclaf Causality Assessment Method instrument is considered the gold standard in causality assessment of hepatotoxicity, although there is probably room for improvement. Current efforts in collecting bona fide cases will make refinements of existing scales feasible. Efforts should also be directed towards the development of an abridged instrument for use in evaluating suspected drug-induced hepatotoxicity at the very beginning of the diagnosis and treatment process when clinical decisions need to be taken. The treatment of idiosyncratic DILI is largely supportive. Early suspicion and withdrawal of the offending agent is the most important therapeutic measure.

DILI: New Insights into Diagnosis and Management

Drug-induced liver injury continues to have a significant impact. With over 1000 drugs now registered as causing DILI, this form of liver injury is the most cited reason for withdrawal of a drug from the marketplace. Despite this, the diagnosis of DILI continues to rely on subjective measures and expert opinion with results that are both difficult to verify and reproduce. However, recent developments in DILI research may provide new insights to improve diagnosis and treatment in the future.

Central role of mitochondria in drug-induced liver injury

A frequent mechanism for drug-induced liver injury (DILI) is the formation of reactive metabolites that trigger hepatitis through direct toxicity or immune reactions. Both events cause mitochondrial membrane disruption. Genetic or acquired factors predispose to metabolite-mediated hepatitis by increasing the formation of the reactive metabolite, decreasing its detoxification, or by the presence of critical human leukocyte antigen molecule(s). In other instances, the parent drug itself triggers mitochondrial membrane disruption or inhibits mitochondrial function through different mechanisms. Drugs can sequester coenzyme A or can inhibit mitochondrial β-oxidation enzymes, the transfer of electrons along the respiratory chain, or adenosine triphosphate (ATP) synthase. Drugs can also destroy mitochondrial DNA, inhibit its replication, decrease mitochondrial transcripts, or hamper mitochondrial protein synthesis. Quite often, a single drug has many different effects on mitochondrial function. A severe impairment of oxidative phosphorylation decreases hepatic ATP, leading to cell dysfunction or necrosis; it can also secondarily inhibit ß-oxidation, thus causing steatosis, and can also inhibit pyruvate catabolism, leading to lactic acidosis. A severe impairment of β-oxidation can cause a fatty liver; further, decreased gluconeogenesis and increased utilization of glucose to compensate for the inability to oxidize fatty acids, together with the mitochondrial toxicity of accumulated free fatty acids and lipid peroxidation products, may impair energy production, possibly leading to coma and death. Susceptibility to parent drug-mediated mitochondrial dysfunction can be increased by factors impairing the removal of the toxic parent compound or by the presence of other medical condition(s) impairing mitochondrial function. New drug molecules should be screened for possible mitochondrial effects.

Review article: Drug-induced liver injury--its pathophysiology and evolving diagnostic tools

BACKGROUND

Drug-induced liver injury (DILI) is a significant cause of morbidity and mortality accounting for at least 13% of acute liver failure cases in the US. It is the leading cause of acute liver failure among patients referred for liver transplantation and the most common reason that drugs in development do not obtain FDA approval. The incidence of DILI has been reported to be one in 10,000 to one in 100,000 patients; however, the actual incidence is probably higher due in part to the difficulty of diagnosis.

AIM

To present a review of the current literature on DILI with a focus on its pathophysiology and evolving diagnostic modalities.

METHODS

A PubMed literature search was conducted using the terms 'drug induced liver injury', 'pathophysiology', 'causality', 'diagnosis', 'toxicogenomics' and 'pharmacogenetics'.

RESULTS

Drug-induced liver injury is an area of ongoing research. From the time it was first recognised, our understanding of the pathophysiology, its classification, diagnosis and reporting by established national networks continues to challenge and evolve. Metabonomics, pharmacogenetics, proteomics and transcriptomics are more recent areas of study that have been applied to further the understanding of DILI.

CONCLUSIONS

Despite recent advances in our understanding of drug-induced liver injury, many aspects of its pathophysiology and clinical impact remain unclear. In addition, genomic-based studies are evolving concepts, which undoubtedly continue to contribute to our understanding of the underlying mechanisms of drug-induced liver injury.

Drug-induced autoimmune-like hepatitis

The clinical phenotype of classical autoimmune hepatitis can be mimicked by idiosyncratic drug-induced liver injury, and differentiation can be difficult. The goals of this review are to enumerate the major agents of drug-induced autoimmune-like hepatitis, describe the clinical findings and risk factors associated with it, detail the clinical tools by which to assess causality, discuss putative pathogenic mechanisms, and describe treatment and outcome. The frequency of drug-induced autoimmune-like hepatitis among patients with classical features of autoimmune hepatitis is 9%. Minocycline and nitrofurantoin are implicated in 90% of cases. Female predominance, acute onset, and absence of cirrhosis at presentation are important clinical manifestations. Genetic factors affecting phase I and phase II transformations of the drug, polymorphisms that protect against cellular oxidative stress, and human leukocyte antigens that modulate the immune response may be important pathogenic components. Clinical judgment is the mainstay of diagnosis as structured diagnostic methods for drug-induced liver injury are imperfect. The covalent binding of a reactive drug metabolite to a hepatocyte surface protein (commonly a phase I or phase II enzyme), formation of a neoantigen, activation of CD8 T lymphocytes with nonselective antigen receptors, and deficient immune regulatory mechanisms are the main bases for a transient loss of self-tolerance. Discontinuation of the offending drug is the essential treatment. Spontaneous improvement usually ensues within 1 month. Corticosteroid therapy is warranted for symptomatic severe disease, and it is almost invariably effective. Relapse after corticosteroid withdrawal probably does not occur, and its absence distinguishes drug-induced disease from classical autoimmune hepatitis.

Mechanisms of drug-induced liver injury: from bedside to bench

The low incidence of idiosyncratic drug-induced liver injury (DILI), together with the lack of a reliable diagnostic biomarker and robust preclinical and in vitro toxicology test systems for the condition have limited our ability to define the mechanisms of DILI. A notable exception is acetaminophen hepatotoxicity, which is associated with the formation of a well-characterized and highly reactive intermediate metabolite, N-acetyl-p-benzoquinone imine. However, studies have also suggested a role for the host immune response and variation in the expression of the lymphocyte CD44 gene in the pathogenesis of acetaminophen hepatotoxicity. A careful review of the laboratory, clinical and histological phenotype of patients with DILI can provide potential clues to the mechanisms of disease pathogenesis, as observed with fialuridine and valproate hepatotoxicity. In addition, the use of transcriptomic and genomic approaches in patients with well-characterized DILI has provided important insights into the involvement of the host immune response in the pathogenesis of hepatotoxicity associated with the administration of flucloxacillin, lumiracoxib or ximelagatran. This Review highlights new developments regarding the potential role of reactive metabolites, mitochondrial toxicity, host immune-response pathways and biliary transporters in the etiopathogenesis of DILI. Going forward, a bedside-to-bench approach could improve our understanding of the mechanisms and risk factors for DILI.

Drug-induced acute liver failure and gastrointestinal complications

The objective of this article is to describe adverse drug events related to the liver and gastrointestinal tract in critically ill patients. PubMed and other resources were used to identify information related to drug-induced acute liver failure, gastrointestinal hypomotility, constipation, diarrhea, gastrointestinal bleeding, and pancreatitis in critically ill patients. This information was reviewed, and data regarding pathophysiology, common drug causes, and guidelines for prevention and management were collected and summarized. In cases in which data in critically ill patients were unavailable, data were extrapolated from other patient populations. Drug-induced acute liver failure can be caused by many drugs routinely used in the intensive care unit and may be associated with significant morbidity and mortality. Drug-related hypomotility and constipation and drug-related diarrhea are reported with many drugs, and these are common adverse drug events in critically ill patients that can substantially complicate the care of these patients. Drug-induced gastrointestinal bleeding and drug-induced pancreatitis occur less frequently, can range in disease severity, and can be associated with morbidity and mortality. Many drugs used in critically ill patients are associated with adverse drug events related to the liver and gastrointestinal tract. Critical care clinicians should be aware of common drug causes of drug-induced acute liver failure, gastrointestinal hypomotility, constipation, diarrhea, gastrointestinal bleeding, and pancreatitis, and should be familiar with the prevention and management of these diverse conditions.

Idiosyncratic drug-induced liver injury and the role of inflammatory stress with an emphasis on an animal model of trovafloxacin hepatotoxicity

Idiosyncratic adverse drug reactions (IADRs) occur in a minority of patients yet account for the majority of postmarketing use restrictions by the Food and Drug Administration. Despite the impact of these toxicities, the underlying mechanisms are still poorly understood. Animal models of IADRs would be beneficial in understanding mechanisms and in developing assays with predictive potential. Recent work exploring the interactions between inflammatory stress and drugs associated with human idiosyncratic drug-induced liver injury (IDILI) has led to the development of the first animal models that apply to a range of drugs. Here, we discuss hypotheses for the mechanisms of IDILI and focus on a murine model of trovafloxacin-induced hepatotoxicity as an example related to the inflammatory stress hypothesis.

Drug-induced autoimmune hepatitis: clinical characteristics and prognosis

Drug-induced autoimmune hepatitis (DIAIH) has been reported to be caused by several drugs. There is a lack of data comparing these patients with other patients with autoimmune hepatitis (AIH). A search was performed using the Mayo Clinic diagnostic medical index for AIH patients and DIAIH patients identified over 10 years. Individuals with overlap syndromes and decompensated liver disease were excluded. Overall, 261 patients (204 females, median age 52) were identified, and 24 (9.2%) were DIAIH cases with a median age of 53 (interquartile range, 24-61). Two drugs, nitrofurantoin (n = 11) and minocycline (n = 11), were the main causes. A similar proportion of DIAIH patients had positive antinuclear antibodies (83% versus 70%) and smooth muscle antibodies (50% versus 45%) as compared with AIH patients. Histological grade and stage were similar in patients with DIAIH versus AIH; however, none of the DIAIH patients had cirrhosis at baseline; this was present in 20% of matched AIH cases. Liver imaging was normal in all minocycline cases. Eight of 11 (73%) nitrofurantoin patients had abnormalities on hepatic imaging (mainly liver atrophy), a finding seen in only 8 of 33 (24%) of a random sample of the rest of the AIH group (P = 0.0089). Corticosteroid responsiveness was similar in DIAIH and the AIH patients. Discontinuation of immunosuppression was tried and successful in 14 DIAIH cases, with no relapses (0%), whereas 65% of the AIH patients had a relapse after discontinuation of immunosuppression (P < 0.0001).

CONCLUSION

A significant proportion of patients with AIH have drug-induced AIH, mainly because of nitrofurantoin and minocycline. These two groups have similar clinical and histological patterns. However, DIAIH patients do not seem to require long-term immunosuppressive therapy.

Mitochondrial involvement in drug-induced liver injury

Mitochondrial dysfunction is a major mechanism of liver injury. A parent drug or its reactive metabolite can trigger outer mitochondrial membrane permeabilization or rupture due to mitochondrial permeability transition. The latter can severely deplete ATP and cause liver cell necrosis, or it can instead lead to apoptosis by releasing cytochrome c, which activates caspases in the cytosol. Necrosis and apoptosis can trigger cytolytic hepatitis resulting in lethal fulminant hepatitis in some patients. Other drugs severely inhibit mitochondrial function and trigger extensive microvesicular steatosis, hypoglycaemia, coma, and death. Milder and more prolonged forms of drug-induced mitochondrial dysfunction can also cause macrovacuolar steatosis. Although this is a benign liver lesion in the short-term, it can progress to steatohepatitis and then to cirrhosis. Patient susceptibility to drug-induced mitochondrial dysfunction and liver injury can sometimes be explained by genetic or acquired variations in drug metabolism and/or elimination that increase the concentration of the toxic species (parent drug or metabolite). Susceptibility may also be increased by the presence of another condition, which also impairs mitochondrial function, such as an inborn mitochondrial cytopathy, beta-oxidation defect, certain viral infections, pregnancy, or the obesity-associated metabolic syndrome. Liver injury due to mitochondrial dysfunction can have important consequences for pharmaceutical companies. It has led to the interruption of clinical trials, the recall of several drugs after marketing, or the introduction of severe black box warnings by drug agencies. Pharmaceutical companies should systematically investigate mitochondrial effects during lead selection or preclinical safety studies.

Current concepts of mechanisms in drug-induced hepatotoxicity

Drug-induced liver injury (DILI) has become a leading cause of severe liver disease in Western countries and therefore poses a major clinical and regulatory challenge. Whereas previously drug-specific pathways leading to initial injury of liver cells were the main focus of mechanistic research and classifications, current concepts see these as initial upstream events and appreciate that subsequent common downstream pathways and their attenuation by drugs and other environmental and genetic factors also have a profound impact on the risk of an individual patient to develop overt liver disease. This review summarizes current mechanistic concepts of DILI in a 3-step model that limits its principle mechanisms to three main ways of initial injury, i.e. direct cell stress, direct mitochondrial impairment, and specific immune reactions. Subsequently, initial injury initiates further downstream events, i.e. direct and death receptor-mediated pathways leading to mitochondrial permeability transition, which then results in apoptotic or necrotic cell death. For all mechanisms, mitochondria play a central role in events leading to apoptotic vs. necrotic cell death. New treatment targets consequently focus on interference with downstream pathways that mediate injury and therefore determine the ultimate outcome of DILI. Genome wide and targeted pharmacogenetic as well as metabonomic approaches are now used in order to reach the key goals of a better understanding of mechanisms in hepatotoxicity, and to develop new strategies for its prediction and treatment. However, the complexity of interactions between genetic and environmental risk factors is considerable, and DILI therefore currently remains unpredictable for most hepatotoxins.

[Clinical characteristics of 159 cases of acute toxic hepatitis]

BACKGROUNDS/AIMS

Toxic hepatitis has recently been discovered to be a major cause of acute hepatitis. We studied the clinical features and prognosis of patients diagnosed with toxic hepatitis at a single institution.

METHODS

A retrospective analysis was performed using medical records of 159 cases of toxic hepatitis that were diagnosed from March 2003 to March 2008. Patients were selected based on a RUCAM score of 4 or above.

RESULTS

The incidence was higher in women (n=97) than in men (n=62). The age (mean+/-SD) of the patients was 51+/-15 years . The major causes of the disease included the use of Korean traditional therapeutic preparations (34.0%), herbal medicines (41.5%), and drugs prescribed by a physician (23.9%). At the time of admission, jaundice was the most common symptom (41.5%), and the results of a liver serum battery were as follows: aspartate aminotransferase, 729.4+/-877.0 IU/L; alanine aminotransferase, 857.1+/-683.0 IU/L; total bilirubin, 6.4+/-6.5 mg/dL; and alkaline phosphatase, 209.8+/-130.0 IU/L. The hospitalization period was 10.0+/-9.5 days, and the duration of recovery from liver injury was 31.0+/-29.5 days. The factors associated with the hospitalization period included the presence of anorexia and the serum levels of albumin and bilirubin at the time of admission (P<0.05). A high serum bilirubin level and a history of alcohol ingestion were associated with a delayed recovery (Plt;0.05). The sex, age, BMI, and duration of medication were not significantly related to the hospitalization and recovery periods.

CONCLUSIONS

The main cause of acute toxic hepatitis in the current study was the use of herbal medicines. The severity of liver injury at the time of admission was a major factor significantly associated with the hospitalization and recovery periods.

Toxicological Significance of Mechanism-Based Inactivation of Cytochrome P450 Enzymes by Drugs

Cytochrome P450 (P450) enzymes oxidize xenobiotics into chemically reactive metabolites or intermediates as well as into stable metabolites. If the reactivity of the product is very high, it binds to a catalytic site or sites of the enzyme itself and inactivates it. This phenomenon is referred to as mechanism-based inactivation. Many clinically important drugs are mechanism-based inactivators that include macrolide antibiotics, calcium channel blockers, and selective serotonin uptake inhibitors, but are not always structurally and pharmacologically related. The inactivation of P450s during drug therapy results in serious drug interactions, since irreversibility of the binding allows enzyme inhibition to be prolonged after elimination of the causal drug. The inhibition of the metabolism of drugs with narrow therapeutic indexes, such as terfenadine and astemizole, leads to toxicities. On the other hand, the fate of P450s after the inactivation and the toxicological consequences remains to be elucidated, while it has been suggested that P450s modified and degraded are involved in some forms of tissue toxicity. Porphyrinogenic drugs, such as griseofulvin, cause mechanism-based heme inactivation, leading to formation of ferrochelatase-inhibitory N-alkylated protoporphyrins and resulting in porphyria. Involvement of P450-derived free heme in halothane-induced hepatotoxicity and catalytic iron in cisplatin-induced nephrotoxicity has also been suggested. Autoantibodies against P450s have been found in hepatitis following administration of tienilic acid and dihydralazine. Tienilic acid is activated by and covalently bound to CYP2C9, and the neoantigens thus formed activate immune systems, resulting in the formation of an autoantibodydirected against CYP2C9, named anti-liver/kidney microsomal autoantibody type 2, whereas the pathological role of the autoantibodies in drug-induced hepatitis remains largely unknown.

Influence of protein-protein interactions on the cellular localization of cytochrome P450

BACKGROUND

Microsomal CYPs are integral membrane proteins that are localized in the endoplasmic reticulum (ER), which is critical for their function. CYPs are co-translationally inserted into the rough ER membrane and are then either directly retained in the smooth ER or retained by a retrieval mechanism or targeted for ER-associated degradation. Protein-protein interactions are likely to be important for proper cellular targeting of CYPs.

OBJECTIVE

Progress in understanding the mechanisms of cellular targeting and ER retention of CYPs is reviewed with emphasis on the role of protein-protein interactions. Possible mechanisms of direct retention are the incorporation of CYPs into an immobile complex in the ER membrane, homooligomerization that prevents inclusion in transport vesicles, exclusion of CYP monomers from transport vesicles or targeting of CYPs to an ER subdomain away from sites of transport vesicle formation. Degradation of CYPs occurs either by lysosomal mechanisms or by the ubiquitin-proteasomal pathway.

METHODS

The scope of this review includes studies published in the research literature that have defined the targeting of CYPs to the ER, the retention of CYPs in the ER and the degradation of CYPs.

RESULTS/CONCLUSION

Targeting of CYPs to the ER is well understood and involves signal recognition particle-mediated delivery to the sec61 complex. The mechanism of ER retention of CYPs remains unclear, but self-oligomerization or binding to large immobile networks do not underlie ER retention of CYPs. An ER retention 'receptor' remains elusive, but BAP31 is important for the proper cellular localization of CYPs and Dap1p is a CYP-binding protein that is a candidate for such a receptor. Identification of protein binding partners of CYPs will be critical to understanding the mechanism of ER retention.

Idiosyncratic drug-induced liver injury: an overview

Drug-induced liver injury (DILI) encompasses a spectrum of clinical disease ranging from mild biochemical abnormalities to acute liver failure. The majority of adverse liver reactions are idiosyncratic, occurring in most instances 5-90 days after the causative medication was last taken. The diagnosis of DILI is clinical, based on history, probability of the suspect medication as a cause of liver injury and exclusion of other hepatic disease. DILI can be hepatocellular (predominant rise in alanine transaminase), cholestatic (predominant rise in alkaline phosphatase) or mixed liver injury. An elevated bilirubin level more than twice the upper limit of normal in patients with hepatocellular liver injury implies severe DILI, with a mortality of approximately 10% and with an incidence rate of 0.7-1.3 per 100,000. Although acute liver failure is rare, 13-17% of all acute liver failure cases are attributed to idiosyncratic drug reactions. Response to drug withdrawal may be delayed up to 1 year with cholestatic liver injury with occasional subsequent progressive cholestasis known as the vanishing bile duct syndrome. Overall, chronic disease may occur in up to 6% even if the offending drug is withdrawn. Antibiotics and NSAIDs are the most common cause of DILI. Statins rarely cause significant liver injury whereas antiretroviral therapy is associated with hepatotoxicity in 10% of treated patients. Multiple mechanisms of DILI have been implicated, including TNF-alpha-activated apoptosis, inhibition of mitochondrial function and neoantigen formation. Risk factors for DILI include age, sex and genetic polymorphisms of drug-metabolising enzymes such as cytochrome P450. In patients with human immunodeficiency virus, the presence of chronic viral hepatitis increases the risk of antiretroviral therapy hepatotoxicity. Over the next decade, the combination of accurate case ascertainment of DILI via clinical networks and the application of genomics and proteomics will hopefully lead to accurate prediction of risk of DILI, so that pharmacotherapy can be optimised with avoidance of adverse hepatic events.

Drug-induced liver injury

Drug-induced liver injury is a frequent cause of hepatic dysfunction. Reliably establishing whether the liver disease was caused by a drug requires the exclusion of other plausible causes and the search for a clinical drug signature. The drug signature consists of the pattern of liver test abnormality, the duration of latency to symptomatic presentation, the presence or absence of immune-mediated hypersensitivity and the response to drug withdrawal. Determination of causality also includes an evaluation of individual susceptibility to drug-induced liver injury. This susceptibility is governed by both genetic and environmental factors. Components of the drug signature in conjunction with certain risk factors have been incorporated into formal scoring systems that are predictive of the likelihood of drug-induced liver injury. The most validated scoring system is the Roussel-Uclaf causality assessment method, which nonetheless retains certain imperfections. Mitigating the potential for drug-induced liver injury is achieved by the identification of toxicity signals during clinical trials and the monitoring of liver tests in clinical practice. There are three signals of liver toxicity in clinical trials: (i) a statistically significant doubling (or more) in the incidence of serum alanine aminotransferase (ALT) elevation >3 x the upper limit of normal (ULN); (ii) any incidence of serum ALT elevation >8-10 x ULN; and (iii) any incidence of serum ALT elevation >3 x ULN accompanied by a serum bilirubin elevation >2 x ULN. Monitoring of liver tests in clinical practice has shown unconvincing efficacy, but where a benefit-risk analysis would favour continued therapy, monthly monitoring may have some benefit compared with no monitoring at all. With rare exception, treatment of drug-induced liver injury is principally supportive. Drug toxicity is the most common cause of acute liver failure, defined as a prolonged prothrombin time (international normalised ratio > or =1.5) and any degree of mental alteration occurring <26 weeks after the onset of illness in a patient without pre-existing cirrhosis. A patient who meets these criteria must be evaluated for liver transplantation. The pathogenesis of drug-induced liver injury can be examined on the basis of the two principal patterns of injury. The hepatocellular pattern is characterised by a predominant rise in the level of transaminases and results from the demise of hepatocytes by means of either apoptosis or necrosis. The cholestatic pattern is characterised by a predominant rise of the serum alkaline phosphatase level and usually results from injury to the bile ductular cells either directly by the drug or its metabolite, or indirectly by an adaptive immune response.

Mechanisms of drug-induced liver disease

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

Immunomodulators, sFas and Fas-L as potential noninvasive predictors of IFN treatment in patients with HCV genotype-4

Recent studies have indicated that cytokines can be used as markers for disease progression in hepatitis C virus (HCV)-infected patients, therefore this study was conducted to determine the influence of pegylated IFN vs standard IFN on interleukin-2 receptor (IL-2R), IL-6R, IL-8, TNFR-I, TNFR-II, sFas, and sFas-L in Egyptian patients with chronic hepatitis C genotype 4, as no previous studies have been performed on this genotype. We also aim at establishing a possible relationship between these cytokines and the response to INF to determine whether they can be used as noninvasive markers for the response to INF therapy and as monitors for the outcome of treatment. Thirty-eight patients with chronic HCV hepatitis were investigated for the serum levels of the previously mentioned cytokines in a randomized opened controlled trial (22 patients treated with pegylated IFN and 16 patients treated with standard IFN). Cytokine levels were measured by ELISA at 0, 1 and 12 months of IFN therapy. There was marked increase in the serum levels of IL-2R and IL-6R in nonresponders to pegylated interferon, IL-8, TNFR-I and II were significantly higher in nonresponders to standard interferon but were also high in responders of pegylated interferon. sFas and sFas-L showed high levels among responders to pegylated interferon but the standard interferon was again less effective in this regard. Serum levels of TNFR-II, sFas and sFas-L have the potential to be used as serological markers for response to pegylated IFN therapy, and IL-8 is a predictor for nonresponse. Moreover, TNFR-I and II have the potential to be used as markers of response to standard IFN treatment. The persistent correlation between sFas and TNFR-II may elaborate the possible role of pegylated IFN in the induction of apoptosis as a possible new mechanism of viral clearance during treatment with pegylated interferon treatment.

Suggested guidelines for patient monitoring: hepatic and hematologic toxicity attributable to systemic dermatologic drugs

Hepatic and hematologic toxicity are among the most fearful adverse effects that occasionally occur as a result of systemic drugs in the dermatologist's therapeutic armamentarium. Drugs of greatest interest concerning hepatic toxicity include methotrexate, azathioprine, dapsone, and acitretin. Somewhat overlapping are drugs that have important hematologic toxicities, including methotrexate, azathioprine, dapsone, sulfonamides, cyclophosphamide, and chlorambucil. Laboratory tests most commonly used include (1) hepatic monitoring: transaminases (AST/SGOT and ALT/SGPT) and the ultrasound-guided liver biopsy, and (2) hematologic monitoring: CBC with diff and platelets along with occasional use of the reticulocyte count. Important principles and specific guidelines for monitoring by drug group are highlighted.

Effect of the CYP2E1 genotype on vinyl chloride monomer-induced liver fibrosis among polyvinyl chloride workers

Although a relationship between vinyl chloride monomer (VCM) and liver cirrhosis has been reported, the underlying mechanisms are not clear. Cytochrome P450 2E1 (CYP2E1), aldehyde dehydrogenase 2 (ALDH2) and glutathione S-transferase theta 1 (GSTT1) enzymes are involved in activation and detoxification of VCM, and thus may be important determinants of interindividual susceptibility to VCM-induced liver damage, including liver cirrhosis. The objective of this study was to evaluate if metabolizing genetic polymorphisms could modify individual susceptibility to liver fibrosis of the VCM exposure. CYP2E1, ALDH2, and GSTT1 polymorphisms were determined by the PCR-RFLP method among 320 workers who were employed in five polyvinyl chloride manufacturing plants. Cumulative VCM exposure levels for study subjects were calculated using a job exposure matrix model. Thirteen workers were diagnosed as having liver fibrosis by using ultrasonography. We observed a dose-response trend between VCM exposure and liver fibrosis. Regarding the results on genetic polymorphisms, CYP2E1 c2c2 genotype showed a significant increase in the risk of liver fibrosis as compared to those with CYP2E1 c1c1 or c1c2 genotypes. No differences were observed between GSTT1 and ALDH2 genotypes and liver fibrosis. In summary, our result suggests that genetic polymorphism in CYP2E1 may be responsible for individual differences in susceptibility to liver fibrosis with regard to chronic VCM exposure. Thus, polymorphism analysis of metabolizing enzymes might be useful in the risk assessment of liver damage in workers with VCM exposure.

Mammalian cytochromes P450—Importance of tissue specificity

Mammals express multiple cytochromes P450 simultaneously in a variety of tissues, including the liver, kidney, lung, adrenal, gonads, brain, and most others. For cytochromes P450 that are expressed in many tissues or cell types, the tissue/cell type-specific expression might be associated with their special physiological roles. Several cytochrome P450 enzymes are found not only in different cell types and tissues, but also in different subcellular compartments. Generally, all mammalian cytochrome P450 enzymes are membrane bound. The two major groups are represented by microsomal cytochromes P450 that reside in the endoplasmic reticulum, and mitochondrial cytochromes P450, that reside in the inner mitochondrial membrane. However, the outer nuclear membrane, different Golgi compartments, peroxisomes and the plasma membrane are also sites where cytochromes P450 were observed. For example, CYP51 is an ER enzyme in majority of tissues but in male germ cells it trafficks through the Golgi to acrosome, where it is stabilized for several weeks. Surprisingly, in brains of heme synthesis deficient mice, a soluble form of CYP1A1 was detected whose activity has been restored by the addition of heme. In the majority of cases each cytochrome P450 enzyme resides in a single subcellular compartment in a certain cell, however, examples of simultaneous localization in different subcellular compartments have also been described, such as endoplasmic reticulum, Golgi and plasma membrane for CYP2E1. This review will focus on the physiological importance of mammalian cytochrome P450 expression and localization in different tissues or cell types and subcellular compartments.

Investigations toward enhanced understanding of hepatic idiosyncratic drug reactions

Idiosyncratic drug reactions (IDRs) of a hepatic origin are a major health concern and a notoriously difficult challenge for the pharmaceutical industry. These types of adverse events are rare, with a typical occurrence of 1 in 100 to 1 in 100,000 patients. Typical adverse outcomes are most likely statistically impossible to predict in traditional preclinical safety studies or clinical trials. Unfortunately, these reactions can pose a significant risk to the public health, resulting in devastating consequences such as irreversible liver injury, liver transplantation and fatality. This review provides many examples of experimental efforts that are underway for a better understanding of molecular events that may be responsible for IDRs. A list of existing hypotheses for IDRs is also provided, each with current literature examples or supporting evidence. The possibilities for developing suitable animal models for the prediction and characterisation of IDRs are elaborated, especially for a drug-inflammation interaction rat model of hepatic IDR. The need for predictive biomarkers of IDR is addressed, with the exploration of some possible candidates. Finally, the use of primary human hepatocyte culture systems is explored as an in vitro system, with application for providing an increased mechanistic knowledge of IDR. Several examples of informative studies on the nature of IDRs that employ toxicogenomic and proteomic technologies are summarised.

Unaltered hepatic oxidative phosphorylation and mitochondrial permeability transition in wistar rats treated with nimesulide: Relevance for nimesulide toxicity characterization

Nonsteroidal anti-inflammatory drugs have been associated with hepatotoxicity in susceptible patients. One such example is nimesulide, a preferential cyclooxygenase 2-inhibitor, widely used for the treatment of inflammation and pain. It was suggested that nimesulide could exert its hepatotoxicity by altering hepatic mitochondrial function, which was demonstrated in vitro. The objective of this study was to verify whether liver mitochondria isolated from rats treated with doses of nimesulide well above therapeutic levels possessed decreased calcium tolerance and oxidative phosphorylation, which indicates in vivo nimesulide mitochondrial toxicity. Male and female rats received nimesulide or its vehicle twice daily, for 5 days, and were killed on the seventh day for the isolation of liver mitochondria. Mitochondrial respiration, transmembrane electric potential, and calcium tolerance were characterized in all experimental groups. Nimesulide had no effect on liver mitochondrial function. Indexes of mitochondrial integrity, calcium loading capacity, and oxidative phosphorylation efficiency were unchanged between liver mitochondria from treated and control animals. In the animals tested, no evidence of degraded mitochondrial function due to nimesulide administration could be found. The results corroborate the notion that despite recognized in vitro mitochondrial toxicity, nimesulide does not cause detectable mitochondrial dysfunction in Wistar rats, even when administered in much higher concentrations than those known to have anti-inflammatory effects.

Ethanol increases mitochondrial cytochrome P450 2E1 in mouse liver and rat hepatocytes

Enhanced hepatic levels of cytochrome P450 2E1 (CYP2E1) may play a key role in the pathogenesis of some liver diseases because CYP2E1 represents a significant source of reactive oxygen species. Although a large fraction of CYP2E1 is located in the endoplasmic reticulum, CYP2E1 is also present in mitochondria. In this study, we asked whether ethanol, a known inducer of microsomal CYP2E1, could also increase CYP2E1 within mitochondria. Our findings indicated that ethanol increased microsomal and mitochondrial CYP2E1 in cultured rat hepatocytes and in the liver of lean mice. This was associated with decreased levels of glutathione, possibly reflecting increased oxidative stress. In contrast, in leptin-deficient obese mice, ethanol administration did not increase mitochondrial CYP2E1, nor it depleted mitochondrial glutathione, suggesting that leptin deficiency hampers mitochondrial targeting of CYP2E1. Thus, ethanol intoxication increases CYP2E1 not only in the endoplasmic reticulum but also in mitochondria, thus favouring oxidative stress in these compartments.

Drug-induced liver injury

Drug-induced hepatotoxicity is a frequent cause of liver injury. The predominant clinical presentation is acute hepatitis and/or cholestasis, although almost any clinical pathological pattern of acute or chronic liver disease can occur. The pathogenesis of drug-induced liver disease usually involves the participation of the parent drug or metabolites that either directly affect the cell biochemistry or elicit an immune response. Each hepatotoxin is associated with a characteristic signature regarding the pattern of injury and latency. However, some drugs may exhibit >1 signature. Susceptibility to drug-induced hepatotoxicity is also influenced by genetic and environmental risk factors. Unpredictable, low-frequency, idiosyncratic reactions often occur on a background of a higher rate of mild asymptomatic liver injury and, although difficult to predict, they may be detected by monitoring serum alanine aminotransferase levels. Recent and future advances in toxicogenomics and proteomics should improve the identification of risk factors and the understanding of idiosyncratic hepatotoxicity.

Genetic and epigenetic factors in autoimmune reactions toward cytochrome P4502E1 in alcoholic liver disease

Autoimmune reactions are often associated with alcoholic liver disease; however, the mechanisms responsible are largely unknown. This study investigates the potential role of the immune response against hydroxyethyl free radical (HER)-derived antigens and of polymorphisms in immunoregulatory genes in the development of anti-cytochrome P4502E1 (CYP2E1) autoantibodies in alcohol abusers. Immunoglobulin G (IgG) recognizing human CYP2E1 and HER-derived epitopes were measured by microplate immunosorbent assay in the sera of 90 patients with alcoholic fibrosis/cirrhosis (ALD), 37 heavy drinkers without liver disease or steatosis only (HD), and 59 healthy subjects. Single nucleotide polymorphisms in the interleukin 10 (IL-10) promoter and in exon 1 of the cytotoxic T-lymphocyte antigen-4 (CTLA-4) gene were genotyped by polymerase chain reaction-restriction fragment length polymorphism analysis. The titers and frequency of anti-CYP2E1 autoantibodies were significantly higher in ALD than in HD subjects or controls. ALD patients with anti-HER IgG had higher titers and a 4-fold increased risk (OR: 4.4 [1.8-10.9]) of developing anti-CYP2E1 autoantibodies than subjects without anti-HER antibodies. The mutant CTLA-4 G allele, but not the IL-10 polymorphism, was associated with an enhanced risk of developing anti-CYP2E1 IgG (OR: 3.8 [1.4-10.3]). CTLA-4 polymorphism did not influence antibody formation toward HER-antigens. ALD patients with concomitant anti-HER IgG and the CTLA-4 G allele had a 22-fold higher (OR: 22.9 [4.2-125.6]) risk of developing anti-CYP2E1 autoreactivity than subjects negative for these factors. In conclusion, antigenic stimulation by HER-modified CYP2E1 combined with an impaired control of T-cell proliferation by CTLA-4 mutation promotes the development of anti-CYP2E1 autoantibodies that might contribute to alcohol-induced liver injury.

Alcohol metabolism: role in toxicity and carcinogenesis

This article contains the proceedings of a symposium at the 2002 RSA Meeting in San Francisco, organized and co-chaired by Thomas M. Badger, Paul Shih-Jiun Yin, and Helmut Seitz. The presentations were (1) First-pass metabolism of ethanol: Basic and clinical aspects, by Charles Lieber; (2) Intracellular CYP2E1 transport, oxidative stress, cytokine release, and ALD, by Magnus Ingelman-Sundberg; (3) Pulsatile ethanol metabolism in intragastric infusion models: Potential role in toxic outcomes, by Thomas M. Badger and Martin J.J. Ronis; (4) Free radicals, adducts, and autoantibodies resulting from ethanol metabolism: Role in ethanol-associated toxicity, by Emanuele Albano; and (5) Gastrointestinal metabolism of ethanol and its possible role in carcinogenesis, by Helmut Seitz.

Regional and cellular expression of CYP2D6 in human brain: higher levels in alcoholics

Cytochrome P450 (CYP) 2D6 is expressed in liver, brain and other extrahepatic tissues where it metabolizes a range of centrally acting drugs and toxins. As ethanol can induce CYP2D in rat brain, we hypothesized that CYP2D6 expression is higher in brains of human alcoholics. We examined regional and cellular expression of CYP2D6 mRNA and protein by RT-PCR, Southern blotting, slot blotting, immunoblotting and immunocytochemistry. A significant correlation was found between mean mRNA and CYP2D6 protein levels across 13 brain regions. Higher expression was detected in 13 brain regions of alcoholics (n = 8) compared to nonalcoholics (n = 5) (anovap < 0.0001). In hippocampus this was localized in CA1-3 pyramidal cells and dentate gyrus granular neurons. In cerebellum this was localized in Purkinje cells and their dendrites. Both of these brain regions, and these same cell-types, are known to be susceptible to alcohol damage. For one case, a poor metabolizer (CYP2D6*4/*4), there was no detectable CYP2D6 protein, confirming the specificity of the antibody used. These data suggest that in alcoholics elevated brain CYP2D6 expression may contribute to altered sensitivity to centrally acting drugs and to the mediation of neurotoxic and behavioral effects of alcohol.

Immune-mediated drug-induced liver disease

Drug-induced immune-mediated hepatic injury is an adverse immune response against the liver that results in a disease with hepatitic, cholestatic, or mixed clinical features. Drugs such as halothane, tienilic acid, dihydralazine, and anticonvulsants trigger a hepatitic reaction, and drugs such as chlorpromazine, erythromycins, amoxicillin-calvulanic acid, sulfonamides and sulindac trigger a cholestatic or mixed reaction. Unstable metabolites derived from the metabolism of the drug may bind to cellular proteins or macromolecules, leading to a direct toxic effect on hepatocytes. Protein adducts formed in the metabolism of the drug may be recognized by the immune system as neoantigens. Immunocyte activation may then generate autoantibodies and cell-mediated immune responses, which in turn damage the hepatocytes. Cytochromes 450 are the major oxidative catalysts in drug metabolism, and they can form a neoantigen by covalently binding with the drug metabolite that they produce. Autoantibodies that develop are selectively directed against the particular cytochrome isoenzyme that metabolized the parent drug. The hapten hypothesis proposes that the drug metabolite can act as a hapten and can modify the self of the individual by covalently binding to proteins. The danger hypothesis proposes that the immune system only responds to a foreign antigen if the antigen is associated with a danger signal, such as cell stress or cell death. Most clinically overt adverse hepatic events associated with drugs are unpredictable, and they have intermediate (1 to 8 weeks) or long latency (up to 12 months) periods characteristic of hypersensitivity reactions. Immune-mediated drug-induced liver disease nearly always disappears or becomes quiescent when the drug is removed. Methyldopa, minocycline, and nitrofurantoin can produce a chronic hepatitis resembling AIH if the drug is continued.

Mechanisms of liver injury relevant to pediatric hepatology

Hepatocyte injury and necrosis from many causes may result in pediatric liver disease. Influenced by other cell types in the liver, by its unique vascular arrangements, by lobular zonation, and by contributory effects of sepsis, reactive oxygen species and disordered hepatic architecture, the hepatocyte is prone to injury from exogenous toxins, from inborn errors of metabolism, from hepatotrophic viruses, and from immune mechanisms. Experimental studies on cultured hepatocytes or animal models must be interpreted with caution. Having discussed general concepts, this review describes immune mechanisms of liver injury, as seen in autoimmune hepatitis, hepatitis B and C infection, the anticonvulsant hypersensitivity syndrome, and autoimmune polyendocrinopathy. Of the monogenic disorders causing significant liver injury in childhood, alpha-1 antitrypsin deficiency and Niemann-Pick C disease demonstrate the effect of endoplasmic or endosomal retention of macromolecules. Tyrosinemia illustrates how understanding the biochemical defect leads to understanding cell injury, extrahepatic porphyric effects, oncogenesis, pharmacological intervention, and possible stem cell therapy. Pathogenesis of cirrhosis in galactosemia remains incompletely understood. In hereditary fructose intolerance, phosphate sequestration causes ATP depletion. Recent information about mitochondrial disease, NASH, disorders of glycosylation, Wilson's disease, and the progressive familial intrahepatic cholestases is discussed.