Halothane hepatitis patients have serum antibodies that react with protein disulfide isomerase

The Immunological Mechanisms and Immune-Based Biomarkers of Drug-Induced Liver Injury

Drug-induced liver injury (DILI) has become one of the major challenges of drug safety all over the word. So far, about 1,100 commonly used drugs including the medications used regularly, herbal and/or dietary supplements, have been reported to induce liver injury. Moreover, DILI is the main cause of the interruption of new drugs development and drugs withdrawn from the pharmaceutical market. Acute DILI may evolve into chronic DILI or even worse, commonly lead to life-threatening acute liver failure in Western countries. It is generally considered to have a close relationship to genetic factors, environmental risk factors, and host immunity, through the drug itself or its metabolites, leading to a series of cellular events, such as haptenization and immune response activation. Despite many researches on DILI, the specific biomarkers about it are not applicable to clinical diagnosis, which still relies on the exclusion of other causes of liver disease in clinical practice as before. Additionally, circumstantial evidence has suggested that DILI is mediated by the immune system. Here, we review the underlying mechanisms of the immune response to DILI and provide guidance for the future development of biomarkers for the early detection, prediction, and diagnosis of DILI.

Idiosyncratic Drug-Induced Liver Injury: Mechanistic and Clinical Challenges

Idiosyncratic drug-induced liver injury (IDILI) remains a significant problem for patients and drug development. The idiosyncratic nature of IDILI makes mechanistic studies difficult, and little is known of its pathogenesis for certain. Circumstantial evidence suggests that most, but not all, IDILI is caused by reactive metabolites of drugs that are bioactivated by cytochromes P450 and other enzymes in the liver. Additionally, there is overwhelming evidence that most IDILI is mediated by the adaptive immune system; one example being the association of IDILI caused by specific drugs with specific human leukocyte antigen (HLA) haplotypes, and this may in part explain the idiosyncratic nature of these reactions. The T cell receptor repertoire likely also contributes to the idiosyncratic nature. Although most of the liver injury is likely mediated by the adaptive immune system, specifically cytotoxic CD8+ T cells, adaptive immune activation first requires an innate immune response to activate antigen presenting cells and produce cytokines required for T cell proliferation. This innate response is likely caused by either a reactive metabolite or some form of cell stress that is clinically silent but not idiosyncratic. If this is true it would make it possible to study the early steps in the immune response that in some patients can lead to IDILI. Other hypotheses have been proposed, such as mitochondrial injury, inhibition of the bile salt export pump, unfolded protein response, and oxidative stress although, in most cases, it is likely that they are also involved in the initiation of an immune response rather than representing a completely separate mechanism. Using the clinical manifestations of liver injury from a number of examples of IDILI-associated drugs, this review aims to summarize and illustrate these mechanistic hypotheses.

Identification of 4-hydroxynonenal protein targets in rat, mouse and human liver microsomes by two-dimensional liquid chromatography/tandem mass spectrometry

RATIONALE

4-Hydroxynonenal (HNE), endogenously generated through peroxidation and breakdown of polyunsaturated fatty acids, has been linked to a number of adverse biological effects through carbonylation of essential biomolecules. Covalent binding of HNE to proteins can alter their structure and functions, causing cell damage as well as adverse immune responses. The liver plays a predominant role in metabolic transformations and hepatic proteins are often targeted by reactive metabolites.

METHODS

Rat, mouse and human liver microsomes were incubated with HNE, enzymatically digested, and subjected to strong cation-exchange peptide fractionation prior to liquid chromatography/tandem mass spectrometry (LC/MS/MS) analysis coupled to electrospray ionization quadrupole time-of-flight (QqTOF) mass spectrometry. HNE-modified peptides were detected by probability-driven peptide spectral matching and comparative analysis between treated and control samples, and confirmed based on accurate mass and high-resolution MS/MS spectra.

RESULTS

A total of 99, 123 and 51 HNE-modified peptides were identified in rat, mouse and human liver microsomes related to 76, 103 and 44 target proteins, respectively. Eight proteins were found to be adducted by HNE in all three species, including ATP synthase, carbamoyl phosphate synthase, cytochrome P450 1A2, glutamate dehydrogenase 1, protein ERGIC-53, protein disulfide-isomerase, and voltage-dependent anion-selective channel protein 1. These proteins play crucial roles in cellular processes and their covalent modification could potentially alter their function and lead to cytotoxicity.

CONCLUSIONS

An analytical approach was developed for the identification of in vitro HNE protein targets in rat, mouse and human liver microsomes using two-dimensional (2D) LC/MS/MS. This approach can be applied to study HNE modification of proteins in vitro and in vivo, providing insight into the toxicology of HNE protein adduction. Copyright © 2016 John Wiley & Sons, Ltd.

Analysis of naphthalene adduct binding sites in model proteins by tandem mass spectrometry

The electrophilic metabolites of the polyaromatic hydrocarbon naphthalene have been shown to bind covalently to proteins and covalent adduct formation correlates with the cytotoxic effects of the chemical in the respiratory system. Although 1,2-naphthalene epoxide, naphthalene diol epoxide, 1,2-naphthoquinone, and 1,4-napthoquinone have been identified as reactive metabolites of interest, the role of each metabolite in total covalent protein adduction and subsequent cytotoxicity remains to be established. To better understand the target residues associated with the reaction of these metabolites with proteins, mass spectrometry was used to identify adducted residues following (1) incubation of metabolites with actin and protein disulfide isomerase (PDI), and (2) activation of naphthalene in microsomal incubations containing supplemental actin or PDI. All four reactive metabolites bound to Cys, Lys or His residues in actin and PDI. Cys₁₇ of actin was the only residue adducted by all metabolites; there was substantial metabolite selectivity for the majority of adducted residues. Modifications of actin and PDI, following microsomal incubations containing ¹⁴C-naphthalene, were detected readily by 2D gel electrophoresis and phosphor imaging. However, target modifications on tryptic peptides from these isolated proteins could not be readily detected by MALDI/TOF/TOF and only three modified peptides were detected using high resolution-selective ion monitoring (HR-SIM). All the reactive metabolites investigated have the potential to modify several residues in a single protein, but even in tissues with very high rates of naphthalene activation, the extent of modification was too low to allow unambiguous identification of a significant number of modified residues in the isolated proteins.

Suppressive and pro-inflammatory roles for IL-4 in the pathogenesis of experimental drug-induced liver injury: a review

IMPORTANCE OF THE FIELD

Idiosyncratic drug reactions resulting in drug-induced liver injury (DILI) account for approximately 13% of acute liver failure cases in the US. Idiosyncratic drug reactions are the third most common cause of liver transplantation, exceeded only by acetaminophen and indeterminate causes. Clinical evidence suggests that idiosyncratic DILI is triggered by drug hapten-altered self proteins resulting in hepatocellular injury. An example of this type of DILI is hepatitis that develops in susceptible individuals following administration of halogenated volatile anesthetics, dihydralazine, carbamazepine or diclofenac.

AREAS COVERED IN THIS REVIEW

In this review, we describe research in animal models that supports a critical role for suppressive and pro-inflammatory roles for IL-4 in the pathogenesis of immune-mediated DILI.

WHAT THE READER WILL GAIN

The reader will gain insights into the roles of IL-4 in the development of experimental DILI. The reader will gain tools to assist in the translation of these findings to those in patients with immune-mediated DILI, as well as other inflammatory diseases of the liver. The reader will then be made aware of gaps in knowledge in the pathogenesis of DILI where research could result in significant advances in the care of these complicated patients.

TAKE HOME MESSAGE

In experimental immune-mediated DILI, IL-4 suppresses regulatory responses to CYP2E1 autoantigens but induces pro-inflammatory responses to drug haptens.

Suppressive and pro-inflammatory roles for IL-4 in the pathogenesis of experimental drug-induced liver injury

The pathogenesis of immune-mediated drug-induced liver injury (DILI) following halogenated anesthetics, carbamazepine or alcohol has not been fully elucidated. Detecting cytochrome P450 2E1 (CYP2E1) IgG4 auto-antibodies in anesthetic DILI patients suggests a role for IL-4 in this hapten-mediated process. We investigated IL-4-mediated mechanisms using our model of experimental DILI induced by immunizing BALB/c (WT) and IL-4(-/-) (KO) mice with S100 liver proteins covalently modified by a trifluoroacetyl chloride (TFA) hapten formed following halogenated anesthetic metabolism by CYP2E1. WT mice developed more hepatitis, TFA and S100 antibodies (p<0.01), as well as T-cell proliferation to CYP2E1 and TFA (p<0.01) than KO mice. Additionally, WT CD4(+) T cells adoptively transferred hepatitis to naïve Rag(-/-) mice (p<0.01). Pro-inflammatory cytokines were expectedly decreased in TFA hapten-stimulated KO splenocyte supernatants (p<0.001); however, IL-2 and IFN-gamma (p<0.05), as well as IL-6 and IL-10 (p<0.001) levels were elevated in CYP2E1-stimulated KO splenocyte supernatants, suggesting dual IL-4-mediated pro-inflammatory and regulatory responses. Anti-IL-10 administered to KO mice increased hepatitis, TFA and CYP2E1 antibodies in KO mice confirming a critical role for IL-4. This is the first demonstration of dual roles for IL-4 in the pathogenesis of immune-mediated DILI by suppressing auto-antigen-induced regulatory responses while promoting hapten-induced pro-inflammatory responses.

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.

Evaluation of Which Reactive Metabolite, If Any, Is Responsible for a Specific Idiosyncratic Reaction

Reactive metabolites are believed to be responsible for most idiosyncratic drug reactions. It is often assumed that if a reactive metabolite is found, it must be responsible for the idiosyncratic reactions associated with that drug. However, the evidence linking reactive metabolites and idiosyncratic reactions is circumstantial at best, and in many cases we have virtually no evidence. Furthermore, it is common for a drug to form several reactive metabolites, so it can be difficult to determine which, if any, is responsible for a given idiosyncratic reaction. Although the reactive metabolite hypothesis is logical, it has important implications for drug development, and we need to develop ways to test the hypothesis for specific drugs rigorously. Valid animal models are a powerful tool for testing whether a specific reactive metabolite is responsible for a specific adverse reaction and for studying further the mechanism by which it may induce such reactions; however, such models are rare.

Idiosyncratic drug reactions: past, present, and future

Although the major working hypothesis for the mechanism of idiosyncratic drug reactions (IDRs), the hapten hypothesis, has not changed since 1987, several hypotheses have been added, for example, the danger hypothesis and the pharmaceutical interaction hypothesis. Genetic studies have found that several IDRs are linked to specific HLA genes, providing additional evidence that they are immune-mediated. Evidence that most IDRs are caused by reactive metabolites has led pharmaceutical companies to avoid drug candidates that form significant amounts of reactive metabolites; however, at least one IDR, ximelagatran-induced liver toxicity, does not appear to be caused by a reactive metabolite. It is possible that there are biomarkers such as those related to cell stress that would predict that a drug candidate would cause a significant incidence of IDRs; however, there has been no systematic study of the changes in gene expression induced by drugs known to cause IDRs. A major impediment to the study of the mechanisms of IDRs is the paucity of valid animal models, and if we had a better mechanistic understanding, it should be easier to develop such models. There is growing evidence that these adverse reactions are more varied and complex than previously recognized, and it is unlikely that a quick fix will be achieved. However, IDRs are an important cause of patient morbidity and mortality and markedly increase the uncertainty of drug development; therefore, continued basic research in this area is essential.

Selection of new chemical entities with decreased potential for adverse drug reactions

Adverse drug reactions, such as hepatotoxicity, blood dyscrasias and hypersensitivity are a major obstacle for the use and the development of new medicines. Many forms of organ-directed toxicity can arise from the bioactivation of drugs to so-called chemically reactive metabolites, which can modify tissue macromolecules. It is well established that the toxicities of model hepatotoxins, such as acetaminophen, furosemide, bromobenzene and methapyrilene can be correlated with the generation of chemically reactive metabolites, which can be detected by measurement of the irreversible binding of radiolabelled material to hepatic protein and/or the detection of stable phase II metabolites such as glutathione conjugates. The basic chemistry of the reaction of such metabolites with model nucleophiles is relatively well understood. A major challenge is to define how certain reactive intermediates may chemically modify critical proteins and how modification of specific amino acids may alter protein function which in turn may affect cell signalling, regulation, defence, function and viability. This in turn will determine whether or not bioactivation will result in a particular form of drug-induced injury. It is now clear that even relatively simple reactive intermediates can react in a discriminative manner with particular cellular proteins and even with specific amino acids within those proteins. Therefore both non-covalent, as well as covalent bonds will be important determinants of the target protein for a particular reactive metabolite. Mammalian cells have evolved numerous defence systems against reactive intermediates. Sensitive redox proteins such as Nrf-2 recognize oxidative stress and electrophilic agents. This is achieved by chemical modification of cysteine groups within keap-1, which normally forms an inactive heterodimer with Nrf-2. Modification of keap-1 releases Nrf-2 that translocates to the nucleus and effects gene transcription of a number of genes involved in the detoxication of chemically reactive metabolites. Diminution of protein function can occur by either covalent modification of nucleophilic amino acids (e.g. cysteine, lysine, histidine etc.) or oxidation of thiols, which can be reversible or irreversible. In the case of acetaminophen, more than 30 target proteins have been identified and for several of them, corresponding alterations in protein function have been defined in the context of tissue necrosis. Alternatively, protein modification may induce signalling systems which initiate cell death, an immune response or to an altered tissue genotype.

Identification of proteins adducted by reactive naphthalene metabolitesin vitro

Metabolic activation of inert chemicals to electrophilic intermediates has been correlated with the incidence and severity of cytotoxicity. The current studies have identified several proteins adducted by reactive metabolites of the lung toxicant, naphthalene. Proteins isolated from microsomal incubations of (14)C-naphthalene were separated by 2-DE, proteins were blotted to PVDF membranes and radioactive proteins were localized by storage phosphor analysis. Adducted proteins were isolated from complimentary gels and identified by peptide mass mapping. A total of 18 adducted proteins were identified including: protein disulfide isomerase precursor, ER-60 protease, alpha actin, mouse urinary proteins, and cytochrome b5 reductase. In supernatant fractions, protein disulfide isomerase, heat shock protein 70, and alpha-actin were key proteins to which reactive naphthalene metabolites were bound. All of the proteins adducted, with the exception of cytochrome b5 reductase were sulfhydryl rich. Although several of the proteins found to be adducted in these studies have also been shown to be adducted by other electrophiles, several others have not been reported as common targets of reactive metabolites. These studies provide a basis for both in situ and in vivo work designed to follow the fate and formation of reactive metabolite protein adducts.

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.

The role of metabolic activation in drug-induced hepatotoxicity

The importance of reactive metabolites in the pathogenesis of drug-induced toxicity has been a focus of research interest since pioneering investigations in the 1950s revealed the link between toxic metabolites and chemical carcinogenesis. There is now a great deal of evidence that shows that reactive metabolites are formed from drugs known to cause hepatotoxicity, but how these toxic species initiate and propagate tissue damage is still poorly understood. This review summarizes the evidence for reactive metabolite formation from hepatotoxic drugs, such as acetaminophen, tamoxifen, diclofenac, and troglitazone, and the current hypotheses of how this leads to liver injury. Several hepatic proteins can be modified by reactive metabolites, but this in general equates poorly with the extent of toxicity. Much more important may be the identification of the critical proteins modified by these toxic species and how this alters their function. It is also important to note that the toxicity of reactive metabolites may be mediated by noncovalent binding mechanisms, which may also have profound effects on normal liver physiology. Technological developments in the wake of the genomic revolution now provide unprecedented power to characterize and quantify covalent modification of individual target proteins and their functional consequences; such information should dramatically improve our understanding of drug-induced hepatotoxic reactions.

Alkylation of protein disulfide isomerase by the episulfonium ion derived from the glutathione conjugate of 1,2-dichloroethane and mass spectrometric characterization of the adducts

The reactivity of the episulfonium ion derived from S-(2-chloroethyl)glutathione (CEG), the glutathione conjugate of 1,2-dichloroethane, with the catalytic sites of protein disulfide isomerase (PDI) was investigated. The two cysteine residues of the two active sites of PDI are expected to be the major targets of alkylation. PDI was incubated with equimolar to 100-fold excess CEG. The activity of PDI was irreversibly inhibited with a concurrent loss of two thiols; however, PDI oxidative refolding activity was not completely inhibited. With mass spectrometry, sequencing PDI identified one alkylation event on each of the N-terminal cysteine residues in the two active site peptides. PDI appears robust and able to maintain some activity by steric constraint. We have established that the episulfonium ion of CEG can adduct PDI and may have important toxicologic significance for 1,2-dichloroethane toxicity.

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.

A toxicologist's guide to biomarkers of hepatic response

Biological markers (biomarkers) are used to recognize, characterize and monitor treatment-related responses following exposure to xenobiotics. Biomarkers serve three primary applications in toxicology: 1) to confirm exposure to a deleterious agent, 2) to provide a system for monitoring individual susceptibility to a toxicant, and 3) to quantitatively assess deleterious effects of a toxicant to an organism or individual. Because the liver is a general target for adverse effects of drugs and other chemicals, biomarkers of untoward hepatic response to xenobiotics are of particular interest to the pharmaceutical toxicologist. General requirements for the latter category of biomarkers are sample availability, target organ specificity, sensitivity for the toxicity of interest, accessibility, a relatively short half-life, and available detection systems. Biomarkers that can be assayed in biological fluids from both human and animal subjects are particularly desirable. Histologically, acute and subacute hepatic toxicity commonly involves necrosis, steatosis, cholestasis, vascular disorders, or multiple lesions. The purpose of this review is to summarize reported applications using clinical analytes and biochemical indicators of hepatic dysfunction with emphasis on those that show promise of supplementing or improving upon standard laboratory procedures. Liver function markers refer to peripheral indicators of hepatic synthetic and secretory activities, enterohepatic function, or perturbations of the hepatic uptake and clearance of circulating biomolecules. Liver injury biomarkers include various peripheral proteins released in response to a cellular damage or locally, proteins that are significantly altered within the liver. These include both circulating cytosolic, mitochondrial, or canalicular membrane markers, and the up-regulation or depletion of radical scavengers, modulators, and stabilizers of intracellular damage. Subsequent recovery from a toxic insult involves repair, regenerative, and proliferative responses that constitute the third class of biomarkers. Of these, protein markers found either in sera, plasma, or urine either during or just prior to the early manifestation of histological hepatic lesions are of greatest interest. Examples of a number of these markers, their documented applications in humans or animals, and potential advantages as well as limitations are presented.

Co-purification of mitochondrial HSP70 and mature protein disulfide isomerase with a functional rat kidney high-M(r) cysteine S-conjugate beta-lyase

S-(1,1,2,2-Tetrafluoroethyl)-L-cysteine (TFEC, the cysteine S-conjugate of tetrafluoroethylene) is an example of a nephrotoxic, halogenated cysteine S-conjugate. Toxicity results in part from the cysteine S-conjugate beta-lyase(s)-catalyzed conversion of TFEC to a thioacylating fragment with the associated production of pyruvate and ammonia. In the present study, we have demonstrated that rat kidney homogenates contain at least three enzyme fractions that are capable of catalyzing a cysteine S-conjugate beta-lyase reaction with TFEC. One of these fractions contains a high-M(r) lyase. At least two proteins co-purify with this high-M(r) complex. N-Terminal analysis (15 cycles) revealed that the smaller species was mature protein disulfide isomerase (M(r) approximately 54,200) from which the 24 amino acid endoplasmic reticulum signal peptide had been removed. Internal amino acid sequencing (15 cycles) revealed that the larger species was mitochondrial HSP70 (mtHSP70; M(r) approximately 75,000). The present findings offer an explanation for the previous observation that mtHSP70 in kidney mitochondria is heavily thioacylated when rats are injected with TFEC (Bruschi et al., J Biol Chem 1993;268:23157-61).

Understanding drug allergies

At this time, the incidence of adverse drug reactions can only be estimated because the intensive monitoring and documenting that is required to make this determination does not exist at most hospitals and clinics. Despite these limitations, a meta-analysis of prospective studies has estimated the incidence of serious adverse drug reactions in hospitalized patients to be 6.7% and the incidence of fatal adverse drug reactions to be 0.32%. When evaluating and managing the condition of a patient who has experienced an adverse drug reaction, the physician first obtains an accurate history and performs a careful physical examination to determine whether the reaction was immunologic in nature. Drug reactions that are immunologically mediated (1) require a period of sensitization, (2) occur in a small proportion of the population, (3) are elicited at drug doses far below the therapeutic range, and (4) subside after drug discontinuation in most instances. All possible culprit drugs should be identified, with dosages and dates of administration and discontinuation, and the patient should be asked about any previous drug exposure history. Although immunodiagnostic tests for allergic drug reactions are limited, several tests do exist and may be useful in the identification of drug-specific antibodies, drug-specific T lymphocytes, or mediators from activated cells. If the reaction was not consistent with an IgE-mediated event and if it did not involve serious organ damage, cautious rechallenge may be considered. For those reactions that appear to be IgE-mediated and for which there is no reliable skin test reagent, drug desensitization may be performed by allergists who are trained in this procedure.

The Long Evans Cinnamon (LEC) rat develops hepatocellular damage in the absence of antimicrosomal antibodies

Long Evans Cinnamon (LEC) rats are an inbred strain with a mutation affecting a copper transporter. As a result, hepatic copper levels rise and the rats spontaneously develop hepatitis that is fatal in about 40% of the rats. The rats that die have been reported to develop anti-microsomal antibodies, most frequently against protein disulfide isomerase (PDI), prior to the onset of liver damage. The association between the presence of antibodies and death of the LEC rats, along with the detection of antibodies prior to the detection of liver damage suggested that the antibodies may have a role in the pathogenesis of liver damage. The objective of this study was to more clearly delineate the temporal relationship between antibody production and the onset of liver damage and copper accumulation. Serum was screened for the presence of anti-microsomal antibodies by immunoblotting. Liver damage was assessed by serum biochemistry and histological examination on rats between 6 and 12 weeks of age (four per group). Copper accumulation in the liver was determined by biochemistry and histological examination. Evidence of liver damage was detectable by serum biochemistry and histopathology by 11.5 weeks. Copper was rarely detected in hepatocytes, although it was detected in macrophages. Sera from only one of seven rats with evidence of liver damage had detectable anti-microsomal protein antibodies. The protein recognized was not PDI. The development of anti-microsomal autoantibodies did not precede the development of significant liver damage, suggesting that they play only a secondary role, if any, in the pathogenesis of hepatitis in this rat strain.

Interindividual variability in P450-dependent generation of neoantigens in halothane hepatitis

Halothane hepatitis occurs because susceptible patients mount immune responses to trifluoroacetylated protein antigens, formed following cytochrome P450-mediated bioactivation of halothane to trifluoroacetyl chloride. In the present study, an in vitro approach has been used to investigate the cytochrome P450 isozyme(s) which catalyze neoantigen formation and to explore the protective role of non-protein thiols (cysteine and reduced glutathione). Significant levels of trifluoroacetyl protein antigens were generated when human liver microsomes, and also microsomes from livers of rats pre-treated with isoniazid, phenobarbital or beta-naphtoflavone, were incubated with halothane plus a nicotinamide adenine dinucleotidephosphate (NADPH) generating system. Immunoblotting studies revealed that the major trifluoroacetyl antigens expressed in vitro exhibited molecular masses of 50-55 kDa and included 60 and 80 kDa neoantigens recognized by antibodies from patients with halothane hepatitis. Much lower concentrations of halothane were required to produce maximal antigen generation in isoniazid-induced rat microsomes, as compared with phenobarbital or isosafrole-induced microsomes (0.5 vs 12.5 microl/ml). In isoniazid-induced microsomes, antigen generation was inhibited > 90% by the nucleophiles cysteine and glutathione and by the CYP2E1-selective inhibitors diallylsulfide and p-nitrophenol, but was unaffected by inhibitors of other P450 isozymes (furafylline, sulfaphenazole or triacetyloleandomycin). Neoantigen formation in six human liver microsomal preparations was inhibited in the presence of diallylsulfide, but not by furafylline, sulfaphenazole or triacetyloleandomycin, and exhibited marked variability which correlated with CYP2E1 levels. These results suggest that the balance between metabolic bioactivation by CYP2E1 and detoxication of reactive metabolites by cellular nucleophiles could be an important metabolic risk factor in halothane hepatitis.

Kupffer cells from halothane-exposed guinea pigs carry trifluoroacetylated protein adducts

The anesthetic, halothane, is bioactivated by the liver cytochrome P450 system to trifluoroacetyl-chloride, which can readily acylate liver protein. Covalent binding of the trifluoroacetyl moiety may result in hapten formation leading to the induction of an immune response and ultimately halothane hepatitis. In this study the presence of trifluoroacetylated-protein adducts in Kupffer cells was investigated to learn how the immune system might come in contact with the proteins. Guinea pigs were exposed to 1.0% halothane, 40% oxygen for 4 h. Kupffer cells were isolated on days 1 through 9 post-exposure, by liver perfusion and purification by elutriation. Using gel electrophoresis and Western blotting techniques, it has been demonstrated that Kupffer cells obtained from halothane-treated guinea pigs, do carry trifluoroacetyl-protein adducts as recognized by an anti-trifluoroacetyl-rabbit serum albumin antibody. Apparent molecular weights of polypeptides bound by trifluoroacetyl were of a wide range, 25-152 kDa. Bands were most prominent in the larger Kupffer cells with more appearing at lower molecular weights. Trifluoroacetyl-protein adducts were not detected in lung, spleen, lymph node or peripheral blood macrophages. This work suggests a role for Kupffer cells in the presentation of altered proteins in the liver to cells of the immune system.

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.

Covalent binding of xenobiotics to specific proteins in the liver

Chemicals that cause toxicity though a direct mechanism, such as acetaminophen, covalently bind to a select group of proteins prior to the development of toxicity, and these proteins may be important in the initiation of the events that lead to the hepatotoxicity. Disruption of the cell is measured by release of intracellular proteins such as alanine aminotransferase and occurs late in the time course following a hepatotoxic dose of a direct toxin. Prior to this disruption, there appears to be a large number of proteins covalently modified by a reactive metabolite. There are at least two possible mechanisms that may cause the toxicity. First, some critical protein is a target of the reactive metabolite. Disruption of the enzymatic function (or a critical pathway for a regulatory protein) may lead directly to cell death. With the direct hepatotoxin acetaminophen, there is a decrease in the activity of several of the early target proteins, but how this disruption of critical proteins leads to the toxicity is still unclear. The early targets appear to be proteins with accessible nucleophilic sulfhydryl groups, and usually the target has a high concentration of the protein within the cell. It is possible that the binding to some of these proteins represents a detoxification protecting more critical targets within the cell. A second mechanism for the direct toxicity is that more and more proteins become targets in the time course following administration of a direct toxin, and eventually the cells machinery is overwhelmed. The cell can then no longer function, or there is a disruption the redox balance within the cell due to the decreased function of numerous proteins. In contrast to the direct-acting toxins, the chemical-protein conjugates that initiate toxicity through an activation of the immune system appear to have a limited number of target proteins and are localized within one subcellular fraction. Halothane produces adducts almost exclusively in the microsomal fraction, and these adducts appear to be limited to selective proteins with high concentrations in this fraction. The substitution level is an important factor in the development of an immune response. Halothane hepatitis patients' antibodies primarily recognize proteins with a high substitution level. For halothane and diclofenac, the proteins are accessible to the immune system through exposure on the plasma membrane. Trichloroethylene binds primarily to a 50-kDa microsomal protein, and preliminary evidence has been presented which indicates that a trichloroethylene-protein conjugate is released into the blood following exposure, where contact with the immune system can occur. In order to elicit an immune response the immune system requires multiple exposure to the chemical-protein conjugates. With halothane hepatitis and with diclofenac hepatitis, as well as occupational and environmental exposure to trichloroethylene, there are multiple exposures leading to repeat presentation of the protein adducts to the immune system; this situation is not generally found with acetaminophen overdose patients. In summary, direct toxicants such as acetaminophen covalently bind to selected targets which may be critical to the development of hepatotoxicity, and they later form adducts with numerous proteins which may overwhelm the cell's capacity to maintain homeostasis, leading to loss of vital function and cell death (Fig.3). In contrast, indirect toxicants that elicit an immune-mediated toxicity such as halothane, and possibly diclofenac and trichloroethylene, appear to have a limited number of protein targets with a high substitution level, and the immune system is exposed repeatedly to the modified proteins.

Secretion, surface localization, turnover, and steady state expression of protein disulfide isomerase in rat hepatocytes

Protein disulfide isomerase in isolated rat hepatocytes was present at a concentration of 7 micrograms/mg cell protein, representing a approximately 2-fold enrichment compared to isolated hepatic non-parenchymal cells. Though localized mainly in microsomal fractions of hepatocytes, direct immunofluorescence and cell surface radioiodination followed by immunoprecipitation revealed the presence of M(r) 57,000 disulfide isomerase at the cell surface. Electrostatic interaction of the protein with the cell surface was suggested by susceptibility to carbonate washing. Metabolic radiolabeling and immunoprecipitation studies also indicated that some of the newly synthesized M(r) 57,000 disulfide isomerase was secreted. Treatment of cells with colchicine markedly reduced the recovery of disulfide isomerase from the media, indicating microtubular-directed secretion of the protein. Partial staphlococcal V8 proteolytic digestion of the secreted protein revealed a peptide pattern similar to that of the cellular protein. Immunoprecipitation with antibody specific to the -KDEL peptide retention sequence confirmed the presence of this sequence in the secreted protein. Studies of the turnover of disulfide isomerase revealed a half-life of approximately 96 h. Treatment of cells with tunicamycin or heat shock resulted in an increased recovery of newly synthesized disulfide isomerase from cell lysates but diminished recovery from the media. The secretion and cell surface distribution of disulfide isomerase in hepatocytes may be important for the pathogenesis of immune mediated liver injury.

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.

Human hepatocytes express trifluoroacetylated neoantigens after in vitro exposure to halothane

Biotransformation of anaesthetic halothane by cytochrome P450-dependent monooxygenases resulted in the production of reactive intermediate trifluoroacetyl (TFA) halide, capable of covalently binding to hepatocyte proteins. TFA-modified liver proteins can act as antigens and are implicated in the pathogenesis of halothane hepatitis in humans. The aim of this study was to investigate the formation of TFA-neoantigens in halothane-treated primary cultures of adult human hepatocytes and to evaluate the usefulness of this in vitro model for studying immune-mediated halothane hepatotoxicity. Cultured human hepatocytes were incubated with halothane under constant temperature, atmosphere and anaesthetic concentration conditions. The results obtained show that halothane-treated hepatocytes isolated from seven different donors produced TFA-antigens as detected by immunocytochemical and western immunoblot analysis using rabbit anti-TFA antiserum. TFA-adducts were localized mainly in the endoplasmic reticulum and in small amounts on the plasma membrane of parenchymal cells. By immunoblotting, several neoantigens, with molecular masses from 42 to 100 kDa, were detected in halothane-exposed hepatocytes. These observations are consistent with the formation of TFA-adducts through metabolism of the anaesthetic and suggest that primary cultures of human hepatocytes represent a suitable in vitro model to study the pathogenesis of immune-mediated halothane hepatotoxicity.

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.