Autoantibodies to hepatic microsomal carboxylesterase in halothane hepatitis

Design, synthesis, and evaluation of a carboxylesterase detection probe with therapeutic effects

In this study, a lysosomal targeting fluorescent probe recognition on CEs was designed and synthesized. The obtained probe BF2-cur-Mor demonstrated excellent selectivity, sensitivity, pH-independence, and enzyme affinity towards CEs within 5 min. BF2-cur-Mor could enable recognition of intracellular CEs and elucidate that the CEs content of different cancer cells follows the rule of HepG2 > HCT-116 > A549 > HeLa, and the CEs expression level of hepatoma cancer cells far exceeds that of normal hepatic cells, being in good agreement with the previous reports. The ability of BF2-cur-Mor to monitor CEs in vivo was confirmed by zebrafish experiment. BF2-cur-Mor exhibits some pharmacological activity in that it can induce apoptosis in hepatocellular carcinoma cells but is weaker in normal hepatocyte cells, being expected to be a potential "diagnostic and therapeutic integration" tool for the clinical diagnosis of CEs-related diseases.

A comprehensive autoantigen-ome of autoimmune liver diseases identified from dermatan sulfate affinity enrichment of liver tissue proteins

BACKGROUND

Autoimmune diseases result from aberrant immune attacks by the body itself. It is mysterious how autoantigens, a large cohort of seemingly unconnected molecules expressed in different parts of the body, can induce similar autoimmune responses. We have previously found that dermatan sulfate (DS) can form complexes with molecules of apoptotic cells and stimulate autoreactive CD5+ B cells to produce autoantibodies. Hence, autoantigenic molecules share a unique biochemical property in their affinity to DS. This study sought to further test this uniform principle of autoantigenicity.

RESULTS

Proteomes were extracted from freshly collected mouse livers. They were loaded onto columns packed with DS-Sepharose resins. Proteins were eluted with step gradients of increasing salt strength. Proteins that bound to DS with weak, moderate, or strong affinity were eluted with 0.4, 0.6, and 1.0 M NaCl, respectively. After desalting, trypsin digestion, and gel electrophoresis, proteins were sequenced by mass spectrometry. To validate whether these proteins have been previously identified as autoantigens, an extensive literature search was conducted using the protein name or its alternative names as keywords. Of the 41 proteins identified from the strong DS-affinity fraction, 33 (80%) were verified autoantigens. Of the 46 proteins with moderate DS-affinity, 27 (59%) were verified autoantigens. Of the 125 proteins with weak DS-affinity, 44 (35%) were known autoantigens. Strikingly, these autoantigens fell into the classical autoantibody categories of autoimmune liver diseases: ANA (anti-nuclear autoantibodies), SMA (anti-smooth muscle autoantibodies), AMA (anti-mitochondrial autoantibodies), and LKM (liver-kidney microsomal autoantigens).

CONCLUSIONS

This study of DS-affinity enrichment of liver proteins establishes a comprehensive autoantigen-ome for autoimmune liver diseases, yielding 104 verified and 108 potential autoantigens. The liver autoantigen-ome sheds light on the molecular origins of autoimmune liver diseases and further supports the notion of a unifying biochemical principle of autoantigenicity.

Autoantibodies in Autoimmune Hepatitis: Can Epitopes Tell Us about the Etiology of the Disease?

Autoimmune hepatitis (AIH), primary biliary cholangitis (PBC), and primary sclerosing cholangitis (PSC) are serious autoimmune liver diseases that are characterized by a progressive destruction of the liver parenchyma and/or the hepatic bile ducts and the development of chronic fibrosis. Left untreated autoimmune liver diseases are often life-threatening, and patients require a liver transplantation to survive. Thus, an early and reliable diagnosis is paramount for the initiation of a proper therapy with immunosuppressive and/or anticholelithic drugs. Besides the analysis of liver biopsies and serum markers indicating liver damage, the screening for specific autoantibodies is an indispensable tool for the diagnosis of autoimmune liver diseases. Such liver autoantigen-specific antibodies might be involved in the disease pathogenesis, and their epitope specificity may give some insight into the etiology of the disease. Here, we will mainly focus on the generation and specificity of autoantibodies in AIH patients. In addition, we will review data from animal models that aim toward a better understanding of the origins and pathogenicity of such autoantibodies.

Severe and fulminant hepatitis of indeterminate etiology in a Japanese center

AIM

The outcome of acute liver failure (ALF) is influenced by its etiology, making etiological consideration of ALF important. However, specific etiology could not be identified in 30-40% of adult patients in a Japanese nationwide survey. We examined our patients with severe (SH) and fulminant hepatitis (FH) of indeterminate etiology for the better understanding of ALF.

METHODS

We investigated 106 adult patients with SH or FH including 24 of indeterminate etiology between 2000 and 2013, retrospectively.

RESULTS

Of 24 patients, 12 were men. Seventeen were SH and seven FH (three FH acute type and four FH subacute type). Eighty-three percent of patients were positive for antinuclear antibody. Seventeen recovered without liver transplantation (LT), two received LT and five died without LT. Histology of 15 patients showed a pattern of acute hepatitis (massive necrosis in four, submassive necrosis in one, severe acute hepatitis in two and acute hepatitis in eight). The involvement of immune-mediated liver injury was histologically suggested in some patients.

CONCLUSION

There was no large cluster of etiology in our patients with indeterminate cause. The causes of ALF of indeterminate etiology were the mixture of various minor or rare ones, if precise diagnosis of acute AIH was done. Outcome of our patients with indeterminate cause was not poor if they were treated as early as possible after the diagnosis of severe disease. Careful examination of unknown viral infection, drugs, toxins, undefined metabolic disorders and histology may help detect some of these etiologies.

Steroid use in acute liver failure

Drug-induced and indeterminate acute liver failure (ALF) might be due to an autoimmune-like hepatitis that is responsive to corticosteroid therapy. The aim of this study was to evaluate whether corticosteroids improve survival in fulminant autoimmune hepatitis, drug-induced, or indeterminate ALF, and whether this benefit varies according to the severity of illness. We conducted a retrospective analysis of autoimmune, indeterminate, and drug-induced ALF patients in the Acute Liver Failure Study Group from 1998-2007. The primary endpoints were overall and spontaneous survival (SS, survival without transplant). In all, 361 ALF patients were studied, 66 with autoimmune (25 steroids, 41 no steroids), 164 with indeterminate (21 steroids, 143 no steroids), and 131 with drug-induced (16 steroids, 115 no steroids) ALF. Steroid use was not associated with improved overall survival (61% versus 66%, P = 0.41), nor with improved survival in any diagnosis category. Steroid use was associated with diminished survival in certain subgroups of patients, including those with the highest quartile of the Model for Endstage Liver Disease (MELD) (>40, survival 30% versus 57%, P = 0.03). In multivariate analysis controlling for steroid use and diagnosis, age (odds ratio [OR] 1.37 per decade), coma grade (OR 2.02 grade 2, 2.65 grade 3, 5.29 grade 4), MELD (OR 1.07), and pH < 7.4 (OR 3.09) were significantly associated with mortality. Although steroid use was associated with a marginal benefit in SS overall (35% versus 23%, P = 0.047), this benefit did not persistent in multivariate analysis; mechanical ventilation (OR 0.24), MELD (OR 0.93), and alanine aminotransferase (1.02) were the only significant predictors of SS.

CONCLUSION

Corticosteroids did not improve overall survival or SS in drug-induced, indeterminate, or autoimmune ALF and were associated with lower survival in patients with the highest MELD scores.

Trichloroethylene hypersensitivity syndrome: a disease of fatal outcome

Trichloroethylene is commonly used as an industrial solvent and degreasing agent. The clinical features of acute and chronic intoxication with trichloroethylene are well-known and have been described in many reports, but hypersensitivity syndrome caused by trichloroethylene is rarely encountered. For managing patients with trichloroethylene hypersensitivity syndrome, avoiding trichloroethylene and initiating glucocorticoid have been generally accepted. Generally, glucocorticoid had been tapered as trichloroethylene hypersensitivity syndrome had ameliorated. However, we encountered a typical case of trichloroethylene hypersensitivity syndrome refractory to high dose glucocorticoid treatment. A 54-year-old Korean man developed jaundice, fever, red sore eyes, and generalized erythematous maculopapular rashes. A detailed history revealed occupational exposure to trichloroethylene. After starting intravenous methylprednisolone, his clinical condition improved remarkably, but we could not reduce prednisolone because his liver enzyme and total bilirubin began to rise within 2 days after reducing prednisolone under 60 mg/day. We recommended an extended admission for complete recovery, but the patient decided to leave the hospital against medical advice. The patient visited the emergency department due to pneumonia and developed asystole, which did not respond to resuscitation.

Autoimmune acute liver failure: proposed clinical and histological criteria

Identifying autoimmune hepatitis as the etiology of acute liver failure (ALF) is potentially important, because administering corticosteroids might avoid the need for liver transplantation. However, clinical and histological criteria of autoimmune ALF (AI-ALF) have not been defined. Liver sections (biopsies and explants) from a 72-patient subset of the ALF Study Group Registry with indeterminate ALF were reviewed by a pathologist blinded to all clinical data and were diagnosed with probable AI-ALF based on four features suggestive of an autoimmune pathogenesis: distinctive patterns of massive hepatic necrosis (present in 42% of sections), presence of lymphoid follicles (32%), a plasma cell-enriched inflammatory infiltrate (63%), and central perivenulitis (65%). Forty-two sections (58%) were considered probable for AI-ALF; this group demonstrated higher serum globulins (3.7 ± 0.2 g/dL versus 3.0 ± 0.2 g/dL; P = 0.037) and a higher prevalence of antinuclear and/or anti-smooth muscle antibodies (73% versus 48%; P = 0.034) compared to those without histology suggestive of probable AI-ALF. Thirty patients concordant for autoantibodies and probable AI-ALF upon histological analysis were more likely to have the classical autoimmune hepatitis phenotype (female predominance [72% versus 48%; P < 0.05], higher globulins [3.9 ± 0.2 g/dL versus 3.0 ± 0.2 g/dL; P < 0.005], and higher incidence of chronic hepatitis in long-term follow-up [67% versus 17%, P = 0.019]) compared to the population without concordant AI-ALF histology and autoantibodies.

CONCLUSION

Patients with indeterminate ALF often have features of autoimmune disease by histological analysis, serological testing, and clinical recurrence during follow-up. In contrast to classical autoimmune hepatitis, histological features of AI-ALF predominate in the centrilobular zone.

LKM-1 sera from autoimmune hepatitis patients that recognize ERp57, carboxylesterase 1 and CYP2D6

Liver kidney microsomal antibody type 1 (LKM-1) is a diagnostic marker for autoimmune hepatitis type 2 (AIH-2). Characterization of LKM autoantibodies of patients with AIH-2 demonstrated that a proportion of LKM sera contains autoantibodies which recognize one or more small linear epitopes on cytochrome P450, CYP2D6, an enzyme of drug metabolism pathways. The identification and epitope mapping of antigens involved in autoimmune diseases are important in understanding the mechanisms triggering autoimmunity and providing guidance for designing immunomodulatory therapy. In this study, several proteins recognized by LKM-1-positive sera in rat and human hepatic microsomes were analyzed by MALDI-TOF-MS after separation with ion-exchange chromatography or two-dimensional polyacrylamide gel electrophoresis. We identified these proteins as ERp57 and carboxylesterase 1 (CES1) as well as CYP2D6. Epitopes in ERp57 and CES1 recognized by LKM-1-positive serum were investigated by enzyme-linked immunosorbent assay (ELISA) with protease-digested peptides of ERp57 and CES1. The peptides comprising amino acids 105-129 of ERp57 and 558-566 of CES1 were specifically recognized by the serum. The epitopes in EPp57 and CES1 recognized by LKM-1-positive sera were homologous with those in hepatitis C virus (HCV). Viral infection of such as HCV may thus possibly trigger autoimmune hepatitis.

Mechanisms, chemical structures and drug metabolism

From the studies that have been done by many laboratories over the last 2 decades, it is now clear that the toxicities produced by many drugs are due to their reactive metabolites. It is though that, in many cases, reactive metabolites cause toxicity by binding covalently to tissue proteins. However, until recently it was difficult to identify these protein targets. Due to the development of an immunochemical approach, this problem has been overcome, as is illustrated here by studies that have been conducted on the metabolic basis of the idiosyncratic hepatitis caused by the inhalation anaesthetic halothane. The major problem to solve in the future will be to determine how protein adduct formation leads to toxicity. It is possible that protein adduct formation may alter an important cellular function or may lead to immunopathology, as is thought to occur in the case of halothane hepatitis. If an allergic reaction is suspected, purified protein targets of reactive metabolites can serve as antigens for identifying sensitized individuals. This information can be used to prevent not only an allergic reaction to the drug, but possible cross-reactions to other drugs that are structurally related. Another important application of these studies is the design of safer alternative drugs that will not produce structurally similar toxic reactive metabolites.

Monitoring drug-protein interaction

A variety of therapeutic drugs can undergo biotransformation via Phase I and Phase II enzymes to reactive metabolites that have intrinsic chemical reactivity toward proteins and cause potential organ toxicity. A drug-protein adduct is a protein complex that forms when electrophilic drugs or their reactive metabolite(s) covalently bind to a protein molecule. Formation of such drug-protein adducts eliciting cellular damages and immune responses has been a major hypothesis for the mechanism of toxicity caused by numerous drugs. The monitoring of protein-drug adducts is important in the kinetic and mechanistic studies of drug-protein adducts and establishment of dose-toxicity relationships. The determination of drug-protein adducts can also provide supportive evidence for diagnosis of drug-induced diseases associated with protein-drug adduct formation in patients. The plasma is the most commonly used matrix for monitoring drug-protein adducts due to its convenience and safety. Measurement of circulating antibodies against drug-protein adducts may be used as a useful surrogate marker in the monitoring of drug-protein adducts. The determination of plasma protein adducts and/or relevant antibodies following administration of several drugs including acetaminophen, dapsone, diclofenac and halothane has been conducted in clinical settings for characterizing drug toxicity associated with drug-protein adduct formation. The monitoring of drug-protein adducts often involves multi-step laboratory procedure including sample collection and preliminary preparation, separation to isolate or extract the target compound from a mixture, identification and determination. However, the monitoring of drug-protein adducts is often difficult because of short half-lives of the protein adducts, sampling problem and lack of sensitive analytical techniques for the protein adducts. Currently, chromatographic (e.g. high performance liquid chromatography) and immunological methods (e.g. enzyme-linked immunosorbent assay) are two major techniques used to determine protein adducts of drugs in patients. The present review highlights the importance for clinical monitoring of drug-protein adducts, with an emphasis on methodology and with a further discussion of the application of these techniques to individual drugs and their target proteins.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Use of molecular simulation for mapping conformational CYP2E1 epitopes

The identification of the epitopes recognized by autoantibodies against cytochrome P450s (CYPs) associated with drug-induced hepatotoxicity is difficult because of their conformational nature. In the present investigation, we used a novel approach based on the analysis of the whole molecule antigenic capacity following single amino acid substitutions to identify the conformational epitopes on CYP2E1. A molecular model of CYP2E1 was generated based on the CYP2C5 crystal structure, and potential motifs for amino acid exchanges were selected by computer simulation in the surface of alpha helices and beta sheets. Fourteen modified, apparently correctly folded CYP2E1 variants were produced in Escherichia coli and evaluated in immunoprecipitation experiments using sera with anti-CYP2E1 autoreactivity from 10 patients with halothane hepatitis and 12 patients with alcoholic liver disease. Ala substitution of Glu-248 and Lys-251 as well as of Lys-324, Lys-342, Lys-420, and Phe-421 severely decreased or abolished CYP2E1 recognition by the majority of both the halothane hepatitis and alcoholic liver disease sera, whereas the other substitutions had only minor effects. Based on the structural model, these substitutions identified two distinct epitopes on the CYP2E1 surface corresponding to the G-helix and an area formed by juxtaposition of the J' and K'' helices, respectively. The combined use of molecular modeling and single amino acid mutagenesis is thus a useful approach for the characterization of conformational epitopes recognized by autoantibodies.

Hepatic effects of halothane, isoflurane or sevoflurane anaesthesia in dogs

The effects of halothane, isoflurane and sevoflurane anaesthesia on hepatic function and hepatocellular damage were investigated in dogs, comparing the activity of hepatic enzymes and bilirubin concentration in serum. An experimental study was designed. Twenty-one clinically normal mongrel dogs were divided into three groups and accordingly anaesthetized with halothane (n = 7), isoflurane (n = 7) and sevoflurane (n = 7). The dogs were 1-4 years old, and weighed between 13.5 and 27 kg (18.4 +/- 3.9). Xylazine HCI (1-2 mg/kg) i.m. was used as pre-anaesthetic medication. Anaesthesia was induced with propofol 2 mg/kg i.v. The trachea was intubated and anaesthesia maintained with halothane, isoflurane or sevoflurane in oxygen at concentrations of 1.35, 2 and 3%, respectively. Intermittent positive pressure ventilation (tidal volume, 15 ml/kg; respiration rate, 12-14/min) was started immediately after intubation and the anaesthesia lasted for 60 min. Venous blood samples were collected before pre-medication, 24 and 48 h, and 7 and 14 days after anaesthesia. Serum level of aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP) and gamma-glutamyltransferase (GGT), lactate dehydrogenase (LDH GGT) activities and bilirubin concentration were measured. Serum AST, ALT and GGT activities increased after anaesthesia in all groups. In the halothane group, serum AST and ALT activities significantly increased all the time after anaesthesia compared with baseline activities. But in the isoflurane group AST and ALT activities increased only between 2 and 7 days, and in the sevoflurane group 7 days after anaesthesia. GGT activity was increased in the halothane group between 2 and 7 days, and in the isoflurane and sevoflurane groups 7 days after anaesthesia. All dogs recovered from anaesthesia without complications and none developed clinical signs of hepatic damage within 14 days. The results suggest that the use of halothane anaesthesia induces an elevation of serum activities of liver enzymes more frequently than isoflurane or sevoflurane from 2 to 14 days after anaesthesia in dogs. The effects of isoflurane or sevoflurane anaesthesia on the liver in dogs is safer than halothane anaesthesia in dogs.

Separation and detection methods for covalent drug–protein adducts

Covalent binding of reactive metabolites of drugs to proteins has been a predominant hypothesis for the mechanism of toxicity caused by numerous drugs. The development of efficient and sensitive analytical methods for the separation, identification, quantification of drug-protein adducts have important clinical and toxicological implications. In the last few decades, continuous progress in analytical methodology has been achieved with substantial increase in the number of new, more specific and more sensitive methods for drug-protein adducts. The methods used for drug-protein adduct studies include those for separation and for subsequent detection and identification. Various chromatographic (e.g., affinity chromatography, ion-exchange chromatography, and high-performance liquid chromatography) and electrophoretic techniques [e.g., sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), two-dimensional SDS-PAGE, and capillary electrophoresis], used alone or in combination, offer an opportunity to purify proteins adducted by reactive drug metabolites. Conventionally, mass spectrometric (MS), nuclear magnetic resonance, and immunological and radioisotope methods are used to detect and identify protein targets for reactive drug metabolites. However, these methods are labor-intensive, and have provided very limited sequence information on the target proteins adducted, and thus the identities of the protein targets are usually unknown. Moreover, the antibody-based methods are limited by the availability, quality, and specificity of antibodies to protein adducts, which greatly hindered the identification of specific protein targets of drugs and their clinical applications. Recently, the use of powerful MS technologies (e.g., matrix-assisted laser desorption/ionization time-of-flight) together with analytical proteomics have enabled one to separate, identify unknown protein adducts, and establish the sequence context of specific adducts by offering the opportunity to search for adducts in proteomes containing a large number of proteins with protein adducts and unmodified proteins. The present review highlights the separation and detection technologies for drug-protein adducts, with an emphasis on methodology, advantages and limitations to these techniques. Furthermore, a brief discussion of the application of these techniques to individual drugs and their target proteins will be outlined.

The role of CYP forms in the metabolism and metabolic activation of HCFCs and other halocarbons

The use of hydrochlorofluorocarbons (HCFCs) such as HCFC-123 (2,2-dichloro-1,1,1-trifluoroethane) and HCFC-141b (1,1-dichloro-1-fluoroethane) is becoming widespread as replacements for the ozone depleting chlorofluorocarbons. Hepatic activation of HCFC-123 or the unsaturated perchloroethylene through oxidative pathways leads to the formation of the electrophiles trifluoroacetyl chloride or trichloroacetyl chloride, respectively. These can react with epsilon-NH(2) functions of lysine in proteins and give rise to neoantigens. In the case of HCFC-123, this reaction is catalysed primarily by CYP2E1 and to a much lesser extent by the constitutive CYP2C19, CYP2B6 and CYP2C8. For perchloroethylene, the extent of activation is less and the reaction is catalysed primarily by the CYP2B family. While acute hepatotoxicity has been seen in humans exposed to HCFC-123 or halothane, little short- or long-term toxicity in rodents is observed. No immunological related toxicity of perchloroethylene has been reported in exposed humans. Long-term exposure of rats can lead to renal tubule carcinomas and in mice, hepatocellular carcinomas. These toxic reactions do not appear to be directly related to the formation of the putative trichloroacetyl chloride intermediate.

Selective protein covalent binding and target organ toxicity

Protein covalent binding by xenobiotic metabolites has long been associated with target organ toxicity but mechanistic involvement of such binding has not been widely demonstrated. Modern biochemical, molecular, and immunochemical approaches have facilitated identification of specific protein targets of xenobiotic covalent binding. Such studies have revealed that protein covalent binding is not random, but rather selective with respect to the proteins targeted. Selective binding to specific cellular target proteins may better correlate with toxicity than total protein covalent binding. Current research is directed at characterizing and identifying the targeted proteins and clarifying the effect of such binding on their structure, function, and potential roles in target organ toxicity. The approaches employed to detect and identify the tartgeted proteins are described. Metabolites of acetaminophen, halothane, and 2,5-hexanedione form covalently bound adducts to recently identified protein targets. The selective binding may influence homeostatic or other cellular responses which in turn contribute to drug toxicity, hypersensitivity, or autoimmunity.

Protein targets of xenobiotic reactive intermediates

Many xenobiotics are metabolically activated to electrophilic intermediates that form covalent adducts with proteins; the mechanism of toxicity is either intrinsic or idiosyncratic in nature. Many intrinsic toxins covalently modify cellular proteins and somehow initiate a sequence of events that leads to toxicity. Major protein adducts of several intrinsic toxins have been identified and demonstrate significant decreases in enzymatic activity. The reactivity of intermediates and subcellular localization of major targets may be important in the toxicity. Idiosyncratic toxicities are mediated through either a metabolic or immune-mediated mechanism. Xenobiotics that cause hypersensitivity/autoimmunity appear to have a limited number of protein targets, which are localized within the subcellular fraction where the electrophile is produced, are highly substituted, and are accessible to the immune system. Metabolic idiosyncratic toxins appear to have limited targets and are localized within a specific subcellular fraction. Identification of protein targets has given us insights into mechanisms of xenobiotic toxicity.

Cytochromes P450 and UDP-glucuronosyl-transferases as hepatocellular autoantigens

Autoantibodies directed against cytochromes P450 or UDP-glucuronosyl-transferases (UGTs) are detected in hepatitis of different aetiology: drug-induced hepatitis autoimmune hepatitis type 2, hepatitis associated with the autoimmune polyglandular syndrome type 1 (APS1) and virus-induced autoimmunity. Autoantibodies directed against cytochrome P450 2C9 are induced by tienilic acid, and anti-P450 1A2 autoantibodies by dihydralazine. Potential mechanisms involved may be metabolic activation of the drugs by cytochromes P450, adduct formation and circumvention of T cell tolerance. In contrast, little is known about the aetiology of autoimmune hepatitis type 2. This disease is characterized by marked female predominance, hypergammaglobulinaemia, circulating autoantibodies and benefit from immunosuppression. Patients with HLA B8, DR3 or DR4 are over-represented. The major target of autoimmunity in this disease is cytochrome P450 2D6. The autoantibodies were shown to be directed against at four short linear epitopes. In addition, about 10% of the patient sera form an additional autoantibody that detects a conformational epitope on UGTs of family 1. The phenomenon of virus-associated autoimmunity is found in chronic infections with hepatitis C and D. In chronic hepatitis C the major target of the autoantibodies again is cytochrome P450 2D6. Some linear and a high proportion of conformational epitopes are recognized. The LKM3 autoantibody is found in 13% of patients with chronic hepatitis D. The target proteins are UGTs of family 1 and, in some sera also, low titres of anto-antibodies directed against UGTs of family 2 are found. The epitopes detected are conformational. In contrast to the patients suffering from autoimmune hepatitis, patients with hepatitis as part of the autoimmune polyglandular syndrome type 1 recognize cytochrome P450 1A2. Interestingly, in APS1 patients also, autoantibodies directed against cytochromes P450 c21, P450 scc and P450 c17a may be detected; these autoantibodies are associated with adrenal and ovarian failure.

Halothane-induced liver injury in guinea-pigs: importance of cytochrome P450 enzyme activity and hepatic blood flow

The basis for susceptibility to halothane-induced liver necrosis in guinea-pigs was examined. In hepatic microsomes, the following were similar in susceptible and resistant animals: total cytochrome (CYP) P450 (P450), phenobarbital-inducible pathways of mixed function oxidation (androstenedione 6 beta- and 16 beta-hydroxylase activities) and the CyP2E1-catalysed pathway of N-nitrosodimethylamine N-demethylase activity. Similarly, immunohistochemical staining of CYP2E1 protein was equivalent in livers from susceptible and resistant guinea-pigs. Prior treatment with the P450-inhibitors, metyrapone and SKF-525A ameliorated halothane-induced liver damage in susceptible animals. Conversely, in resistant guinea-pigs, stimulation of hepatic CYP2E1 activity by treatment with 4-methylpyrazole produced severe hepatotoxicity after re-exposure to halothane. These results confirm the conclusions of others, that P450-mediated metabolism produces halothane-induced liver necrosis in the guinea-pig model but, as in other work, the data fail to explain why no difference in activity of these enzymes could be found between susceptible and resistant guinea-pigs. To establish whether a differential effect on hepatic blood flow between susceptible and resistant guinea-pigs could explain this paradox, studies were performed using a radiolabelled microsphere technique. The effect of halothane on lowering cardiac output was identical in both groups of animals and halothane significantly reduced hepatic arterial but not portal blood flow. The effect on arterial blood flow was more profound in susceptible guinea-pigs (0.67 +/- 0.17% of injected microspheres) than in resistant animals (0.99 +/- 0.13%; P < 0.005). It is concluded that P450-catalysed metabolism and reduced hepatic blood flow are both necessary to produce halothane-induced liver injury in susceptible guinea-pigs, but it is the effect of halothane on hepatic arterial blood flow that differs between susceptible and resistant animals.

Hepatitis, rash and eosinophilia following trichloroethylene exposure: a case report and speculation on mechanistic similarity to halothane induced hepatitis

CASE REPORT

A previously healthy 30-year-old male began work as a degreaser. The solvent used in the degreasing operation was trichloroethylene. Over the next month he experienced symptoms of weakness, dizziness, decreased appetite, nausea, abdominal pain, diarrhea, fever, chills, dry skin, red rash with bumps, peeling face, and itching. At that time he had marked liver enzyme elevation without evidence of cholestasis. CBC was remarkable for a significant number of atypical lymphocytes. Two weeks later his liver enzymes showed a marked reduction in ALT from a peak of 1250 IU to 717 IU. Tests for Hepatitis A, B, and C, CMV, HIV1 were all negative. The night following his first day back at work he had a recurrence of a red, diffuse rash without any consumption of alcohol. The rash caused tremendous itching. Over the next few days off work the rash continued and peeled. Physical examination one week after re-exposure was remarkable for diffuse, erythematous rash; some peeling skin and pitting edema to the knees. ALT was 517 IU/L. White blood cell count was 10,100/mm3 with 27% eosinophilia.

CONCLUSION

This patient had possibly experienced sensitization to trichloroethylene, or more likely, to one of its metabolites. Similar symptoms attributed to trichloroethylene have been reported in only a few other patients. Patch testing with trichloroethylene and its metabolites may better clarify a causal relationship in future patients. If an immune mechanism is involved it may be similar to one postulated for halothane induced hepatitis.

Molecular mimicry in halothane hepatitis: biochemical and structural characterization of lipoylated autoantigens

Exposure of human individuals to halothane causes, in about 20% of all cases, a mild transient form of hepatotoxicity. A small subset of exposed individuals, however, develops a potentially severe and life-threatening form of hepatic damage, coined halothane hepatitis. Halothane hepatitis is thought to have an immunological basis. Sera of afflicted individuals contain a wide variety of autoantibodies against hepatic proteins, in both trifluoroacetylated form (CF3CO-proteins) and, at least in part, in native form. CF3CO-proteins are elicited in the course of oxidative biotransformation of halothane, and include the trifluoroacetylated forms of protein disulfide isomerase, microsomal carboxylesterase, calreticulin, ERp72, GRP 78, and ERp99. Current evidence suggests that CF3CO-proteins arise in all halothane-exposed individuals; however, the vast majority of individuals appear to immunochemically tolerate CF3CO-proteins. The lack of immunological responsiveness of these individuals towards CF3CO-proteins might be due to tolerance, induced through the occurrence of structures in the repertoire of self-determinants, which immunochemically and structurally mimic CF3CO-proteins very closely. In fact, lipoic acid, the prosthetic group of the constitutively expressed E2 subunits of the family of mammalian 2-oxoacid dehydrogenase complexes and of protein X, was shown by immunochemical and molecular modelling analysis to be a perfect structural mimic of N6-trifluoroacetyl-L-lysine (CF3 CO-Lys), the major haptenic group of CF3CO-proteins. As a consequence of molecular mimicry, autoantibodies in patients' sera not only recognize CF3CO-proteins, but also the E2 subunit proteins of the 2-oxoacid dehydrogenase complexes and protein X, as autoantigens associated with halothane hepatitis. Furthermore, a fraction of patients with halothane hepatitis exhibit irregularities in the hepatic expression levels of these native, not trifluoroacetylated autoantigens. Collectively, these data suggest that molecular mimicry of CF3CO-Lys by lipoic acid, or the impairment thereof, might play a role in the susceptibility of individuals for the development of halothane hepatitis.

Immunohistochemistry with an antibody to human liver carboxylesterase in human brain tissues

Human liver carboxylesterase (CE) is an enzyme capable of metabolizing drugs, and may also function as a regulator of lipid metabolism. We examined one isoform of CE by immunohistochemistry in the brains of neurologically normal, Alzheimer disease (AD), amyotrophic lateral sclerosis (ALS) and cerebral infarction cases. In all but the infarcted brains, the anti-CE antibody stained only capillary endothelial cells in the brain and spinal cord tissues. In infarct brain areas, intense immunoreactivity of the macrophages was seen. In contrast, the macrophages in the ALS lateral columns and the reactive microglia located in the center of classical senile plaques in AD, as well as other reactive microglial cells in the grey matter, showed no immunoreactivity. In the central nervous system, CE may function as a protective factor against foreign chemicals in capillary endothelial cells, and the antibody to CE may serve as a marker for invading macrophages from the systemic circulation.

Formation of trifluoroacetylated protein antigens in cultured rat hepatocytes exposed to halothane in vitro

Immune responses to novel, halothane metabolite-modified protein antigens (tri-fluoroacetylated proteins; TFA-proteins) have been implicated in the pathogenesis of halothane hepatitis. The aim of the present study was to investigate and characterize expression of TFA-proteins in cultures of rat hepatocytes which were exposed to halothane in vitro. Following exposure to halothane, the hepatocytes were harvested, then subcellular fractions were prepared and were analysed by immunoblotting for expression of antigens recognized by a rabbit anti-TFA antiserum, and by antibodies in sera from two patients with halothane hepatitis. Hepatocytes exposed to halothane in vitro were shown to express novel microsomal protein antigens, which exhibited molecular masses that were identical to the molecular masses of the major TFA-protein antigens expressed in vivo, in livers of halothane-treated rats (100, 80 and 60 kDa). Experiments in which hepatocytes were exposed to halothane in the presence of SKF-525A, or were exposed to deuterated halothane in place of halothane, confirmed that these novel antigens were TFA-modified proteins whose generation required cytochrome P450-mediated metabolism of halothane. The maximal levels of TFA-antigens expressed in vitro were about 30% of the levels expressed in halothane-treated rats in vivo. Maximal expression of the TFA-antigens in vitro occurred when hepatocytes were exposed to halothane at doses which yielded concentrations of the drug in culture medium of about 13 microM. Expression of the antigens in vitro occurred slowly, with an apparent half-time of about 8 hr. Overall, these results demonstrate that the properties of the TFA-antigens expressed in cultured hepatocytes in vitro closely resemble the properties exhibited by the antigens expressed in vivo, in livers of halothane-treated rats.

Identification of the dihydrolipoamide acetyltransferase subunit of the human pyruvate dehydrogenase complex as an autoantigen in halothane hepatitis. Molecular mimicry of trifluoroacetyl-lysine by lipoic acid

Trifluoroacetylated (CF3CO-) proteins, elicited upon exposure of animals or humans to halothane, were recognized by anti-CF3CO antibody, monospecific for the hapten derivative N6-trifluoroacetyl-L-lysine. Anti-CF3CO antibodies cross-reacted with the dihydrolipoamide acetyltransferase (E2 subunit) of pyruvate dehydrogenase, indicating that epitopes on the E2 subunit of pyruvate dehydrogenase molecularly mimic those on CF3CO-proteins. Lipoic acid, the prosthetic group of the E2 subunit of pyruvate dehydrogenase was essential in this process, in that only the lipoylated form of the recombinantly expressed inner lipoyl domain of the human E2 subunit of pyruvate dehydrogenase, but not the unlipolyated form, was recognized by anti-CF3CO antibody. Furthermore, based on a high degree of structural relatedness, both CF3CO-Lys and (6RS)-lipoic acid, as well as the lipoylated peptide ETDK(lipoyl)ATIG specifically inhibited the recognition by anti-CF3CO antibody of the E2 subunit of pyruvate dehydrogenase, of trifluoroacetylated rabbit serum albumin and of human liver CF3CO-proteins. In sera of patients with halothane hepatitis, autoantibodies with properties identical to those of anti-CF3CO antibody were identified which could not discriminate between CF3CO-proteins and the E2 subunit of pyruvate dehydrogenase. These data suggest that the E2 subunit pyruvate of dehydrogenase is an autoantigen in halothane hepatitis and that molecular mimicry of CF3CO-proteins by the E2 subunit of pyruvate dehydrogenase is due to the similar structures of CF3CO-Lys and lipoic acid.