Presence of functional cytochrome P-450 on isolated rat hepatocyte plasma membrane

Downregulation of Glutathione-Mediated Detoxification Capacity by Binge Drinking Aggravates Acetaminophen-Induced Liver Injury through IRE1α ER Stress Signaling

Overdose of acetaminophen (APAP) can cause severe liver injury. Although alcohol is considered a risk factor for APAP toxicity, the mechanism underlying the interaction between alcohol and APAP remains unclear. Binge alcohol (5 g/kg every 12 h, 3 doses) reduced the concentration of cysteine and glutathione (GSH) and decreased expression of cystathionine β-synthase (CβS), cystathionine γ-lyase (CγL), and glutamate cysteine ligase catalytic subunit (GCLC) in the livers of male C57BL/6 mice. Furthermore, the levels of GSH S-transferase (GST) and GSH peroxidase (GPx) were decreased. To evaluate the effect of binge drinking on APAP-induced liver injury, 300 mg APAP was administered following alcohol binges. APAP in the binge group significantly amplified the serum ALT more than two fold and enhanced the pro-apoptotic proteins with a severe centrilobular necrosis compared to APAP alone. APAP treatment after alcohol binges caused lower levels of hepatic cysteine and GSH than APAP alone over 24 h, indicating that alcohol binges reduced GSH regenerating potential. Exposure to APAP after binge treatment significantly increased oxidative stress (lipid peroxidation) and endoplasmic reticulum (ER) stress (Grp78 and ATF6) markers at 6 h after treatment. Notably, the IRE1α/ASK1/MKK4/JNK pathway was activated, whereas CHOP expression was reduced by APAP administration in mice with pre-exposed alcohol binges compared with APAP alone. Thus, pretreatment with binge alcohol decreases GSH-mediated antioxidant capacity and contributes to augmentation of liver injury caused by subsequent APAP administration through differential ER stress signaling pathway.

CYP2E1 in Alcoholic and Non-Alcoholic Liver Injury. Roles of ROS, Reactive Intermediates and Lipid Overload

CYP2E1 is one of the fifty-seven cytochrome P450 genes in the human genome and is highly conserved. CYP2E1 is a unique P450 enzyme because its heme iron is constitutively in the high spin state, allowing direct reduction of, e.g., dioxygen, causing the formation of a variety of reactive oxygen species and reduction of xenobiotics to toxic products. The CYP2E1 enzyme has been the focus of scientific interest due to (i) its important endogenous function in liver homeostasis, (ii) its ability to activate procarcinogens and to convert certain drugs, e.g., paracetamol and anesthetics, to cytotoxic end products, (iii) its unique ability to effectively reduce dioxygen to radical species causing liver injury, (iv) its capability to reduce compounds, often generating radical intermediates of direct toxic or indirect immunotoxic properties and (v) its contribution to the development of alcoholic liver disease, steatosis and NASH. In this overview, we present the discovery of the enzyme and studies in humans, 3D liver systems and genetically modified mice to disclose its function and clinical relevance. Induction of the CYP2E1 enzyme either by alcohol or high-fat diet leads to increased severity of liver pathology and likelihood to develop ALD and NASH, with subsequent influence on the occurrence of hepatocellular cancer. Thus, fat-dependent induction of the enzyme might provide a link between steatosis and fibrosis in the liver. We conclude that CYP2E1 has many important physiological functions and is a key enzyme for hepatic carcinogenesis, drug toxicity and liver disease.

Circulating extracellular vesicles are a biomarker for NAFLD resolution and response to weight loss surgery

There is increasing interest in the development of minimally invasive biomarkers for the diagnosis and prognosis of NAFLD via extracellular vesicles (EV). Plasma EVs were isolated by differential ultracentrifugation and quantified by nanoparticle tracking analysis from pre (n = 28) and post (n = 28) weight loss patients. In the pre weight loss group 22 had NAFLD. Nanoplasmon enhanced scattering (nPES) of gold nanoparticles conjugated to hepatocyte-specific antibodies was employed to identify hepatocyte-specific EVs. Complex lipid panel and targeted sphingolipids were performed. Logistic regression analysis was used to identify predictors of NAFLD. Plasma levels of EVs and hepatocyte-derived EVs are dynamic and decrease following NAFLD resolution due to weight loss surgery. Hepatocyte-derived EVs correlate with steatosis in NAFLD patients and steatosis and inflammation in NASH patients. Plasma levels of small EVs correlate with EV sphingolipids in patients with NASH. Hepatocyte-derived EVs measured by the nPES assay could serve as a point-of-care test for NAFLD.

Cytochrome P450 2E1 (CYP2E1) positively regulates lipid catabolism and induces browning in 3T3-L1 white adipocytes

AIMS

Browning induction (beiging) of white adipocytes is an emerging prospective strategy to defeat obesity and its related metabolic disorders. Cytochrome P450 2E1 (CYP2E1), a membrane protein which belongs to the cytochrome P450 superfamily, reportedly functions in the xenobiotic metabolism in the body, especially ethanol metabolism. Although previous studies have reported the effect of CYP2E1 on obesity in animal models, the data remains controversial. In the current study, we investigate for the first time, the role of CYP2E1 in lipid metabolism in 3T3-L1 white adipocytes, with a focus on fat browning.

METHODS

3T3-L1 white adipocytes and Cyp2e1 siRNA were applied to investigate the role of CYP2E1 in white adipocytes. After that, cells were seperately exposed to β3-AR agonist, β3-AR antagonist and p38 inhibitor to identify the pathway which CYP2E1 was involved in to regulate browning event in white adipocytes.

KEY FINDINGS

We found that CYP2E1 deficiency results in reduced adipogenesis and lipogenesis as well as brown adipocyte-like phenotype induction. A mechanistic study to identify the molecular signals for CYP2E1 regulation in the browning of white adipocytes revealed that CYP2E1 inhibition deters the β3-adrenergic receptor activation and its downstream targets.

SIGNIFICANCE

Our data unveilved a previously unknown mechanism in the regulation of browning by CYP2E1 in 3T3-L1 white adipocytes, suggesting that CYP2E1 is a promising molecular target for the treatment of obesity and its related diseases.

Characterization of Cellular Sources and Circulating Levels of Extracellular Vesicles in a Dietary Murine Model of Nonalcoholic Steatohepatitis

Circulating extracellular vesicles (EVs) are a novel and emerging biomarker for nonalcoholic steatohepatitis (NASH). It has been demonstrated that total circulating EVs and hepatocyte‐derived EVs are elevated in male mice with diet‐induced NASH. How hepatocyte‐derived EVs change over time and other cellular sources of EVs in NASH have not been determined. Our objective was to define the quantitative evolution of hepatocyte‐derived, macrophage‐derived, neutrophil‐derived, and platelet‐derived EVs in male and female mice with dietary NASH. Fluorescently labeled antibodies and a nanoscale flow cytometer were used to detect plasma levels of EVs. Asialoglycoprotein receptor 1 (ASGR1) and cytochrome P450 family 2 subfamily E member 1 (CYP2E1) are markers of hepatocyte‐derived EVs; galectin 3 is a marker of macrophage‐derived EVs; common epitope on lymphocyte antigen 6 complex, locus G/C1 (Ly‐6G and Ly‐6C) is a marker of neutrophil‐derived EVs; and clusters of differentiation 61 (CD61) is a marker of platelet‐derived EVs. Nonalcoholic fatty liver disease activity score (NAS) was calculated using hematoxylin and eosin‐stained liver sections, and magnetic resonance imaging (MRI) was used for measurement of the fat fraction and elastography. Hepatocyte‐derived EVs increased in both male and female mice at 12 and 10 weeks of feeding, respectively, and remained elevated at 24 weeks in both male and female mice and at 48 weeks in male mice and 36 weeks in female mice. Macrophage‐ and neutrophil‐derived EVs were significantly elevated at 24 weeks of dietary feeding concomitant with the histologic presence of inflammatory foci in the liver. In fat‐, fructose‐, and cholesterol‐ (FFC) fed male mice, platelet‐derived EVs were elevated at 12, 24, and 48 weeks, whereas in female mice, platelet derived EVs were significantly elevated at 24 weeks. Hepatocyte‐, macrophage‐ and neutrophil‐derived EVs correlated well with the histologic NAS. Conclusion: Circulating cell‐type‐specific EVs may be a novel biomarker for NASH diagnosis and longitudinal follow up.

Plasma membrane localization of CYP4Z1 and CYP19A1 and the detection of anti-CYP19A1 autoantibodies in humans

It is thought that autoantibody (aAb) production can be caused by (aberrant) protein targeting to the plasma surface of cells. We recently demonstrated the presence of the human cytochrome P450 enzyme CYP4Z1 on the plasma membrane of MCF-7 breast cancer cells and the detection of high titers of anti-CYP4Z1 aAbs in breast cancer patients, but not in healthy controls. In the present study we show that cells of the normal breast cell line MCF-10A do not display CYP4Z1 on their surface. By contrast, we detected CYP19A1 (aromatase) on the plasma membrane of both cell lines. Interestingly, the presence of CYPs on the cell surface did not correlate with their relative expression levels in these cell lines. Indirect ELISA experiments demonstrated the presence of anti-CYP19A1 aAbs in female breast cancer patient sera as well as in male and female controls, respectively; aAb titers in all three groups varied considerably and overall, the results obtained for each group were not significantly different from those of either of the other two groups. Based on these data we propose the hypothesis that CYP translocation to the plasma membrane, but not the intracellular expression level, is the crucial precondition for the generation of anti-CYP aAbs.

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.

Molecular mechanisms of the microsomal mixed function oxidases and biological and pathological implications

The cytochrome P450 mixed function oxidase enzymes play a major role in the metabolism of important endogenous substrates as well as in the biotransformation of xenobiotics. The liver P450 system is the most active in metabolism of exogenous substrates. This review briefly describes the liver P450 (CYP) mixed function oxidase system with respect to its enzymatic components and functions. Electron transfer by the NADPH-P450 oxidoreductase is required for reduction of the heme of P450, necessary for binding of molecular oxygen. Binding of substrates to P450 produce substrate binding spectra. The P450 catalytic cycle is complex and rate-limiting steps are not clear. Many types of chemical reactions can be catalyzed by P450 enzymes, making this family among the most diverse catalysts known. There are multiple forms of P450s arranged into families based on structural homology. The major drug metabolizing CYPs are discussed with respect to typical substrates, inducers and inhibitors and their polymorphic forms. The composition of CYPs in humans varies considerably among individuals because of sex and age differences, the influence of diet, liver disease, presence of potential inducers and/or inhibitors. Because of such factors and CYP polymorphisms, and overlapping drug specificity, there is a large variability in the content and composition of P450 enzymes among individuals. This can result in large variations in drug metabolism by humans and often can contribute to drug–drug interactions and adverse drug reactions. Because of many of the above factors, especially CYP polymorphisms, there has been much interest in personalized medicine especially with respect to which CYPs and which of their polymorphic forms are present in order to attempt to determine what drug therapy and what dosage would reflect the best therapeutic strategy in treating individual patients.

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The CYP P450 system is important in metabolism of endogenous substrates and drugs.

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About 150 forms of CYPs have been identified and they are grouped into families.

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CYPs catalyze a wide variety of reactions and are among the most diverse catalysts known.

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Electrons are passed to the CYP via NADPH+NADPH-cytochrome P450 reductase.

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Metabolism of certain compounds by CYPs generate reactive intermediates which are toxic.

CYP2E1 autoantibodies in liver diseases

Autoimmune reactions involving cytochrome P4502E1 (CYP2E1) are a feature of idiosyncratic liver injury induced by halogenated hydrocarbons and isoniazid, but are also detectable in about one third of the patients with advanced alcoholic liver disease (ALD) and chronic hepatitis C (CHC). In these latter the presence of anti-CYP2E1 auto-antibodies is an independent predictor of extensive necro-inflammation and fibrosis and worsens the recurrence of hepatitis following liver transplantation, indicating that CYP2E1-directed autoimmunity can contribute to hepatic injury. The molecular characterization of the antigens recognized by anti-CYP2E1 auto-antibodies in ALD and CHC has shown that the targeted conformational epitopes are located in close proximity on the molecular surface. Furthermore, these epitopes can be recognized on CYP2E1 expressed on hepatocyte plasma membranes where they can trigger antibody-mediated cytotoxicity. This does not exclude that T cell-mediated responses against CYP2E1 might also be involved in causing hepatocyte damage. CYP2E1 structural modifications by reactive metabolites and molecular mimicry represent important factors in the breaking of self-tolerance against CYP2E1 in, respectively, ALD and CHC. However, genetic or acquired interferences with the mechanisms controlling the homeostasis of the immune system are also likely to contribute. More studies are needed to better characterize the impact of anti-CYP2E1 autoimmunity in liver diseases particularly in relation to the fact that common metabolic alterations such as obesity and diabetes stimulates hepatic CYP2E1 expression.

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CYP2E1 is a frequent autoimmune target in alcoholic liver disease and hepatitis C.

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Anti-CYP2E1 auto-antibodies mainly target conformational epitopes.

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Molecular mimicry contribute to anti-CYP2E1 autoimmunity during HCV infection.

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Anti-CYP2E1 autoimmunity contributes to the evolution of liver damage.

Central role of mitochondria in drug-induced liver injury

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

Role of oxidative stress in alcohol-induced liver injury

Reactive oxygen species (ROS) are highly reactive molecules that are naturally generated in small amounts during the body's metabolic reactions and can react with and damage complex cellular molecules such as lipids, proteins, or DNA. Acute and chronic ethanol treatments increase the production of ROS, lower cellular antioxidant levels, and enhance oxidative stress in many tissues, especially the liver. Ethanol-induced oxidative stress plays a major role in the mechanisms by which ethanol produces liver injury. Many pathways play a key role in how ethanol induces oxidative stress. This review summarizes some of the leading pathways and discusses the evidence for their contribution to alcohol-induced liver injury. Special emphasis is placed on CYP2E1, which is induced by alcohol and is reactive in metabolizing and activating many hepatotoxins, including ethanol, to reactive products, and in generating ROS.

Increased immunogenicity to P815 cells modified with malondialdehyde and acetaldehyde

Aldehyde modified proteins have been associated with the development and/or progression of alcoholic liver disease (ALD). These protein adducts are capable of initiating many immunological responses that are harmful to the normal homeostasis of organism function. Previous studies have shown that malondialdehyde (MDA) and acetaldehyde (AA) synergistically form a unique adduct (MAA) with soluble proteins, which are capable of inducing cytokine release, T-cell proliferation, and antibody production. The purpose of this study was to determine whether MAA adduction can elicit similar responses to cells using a well-defined tumor model. The mouse mastocytoma P815 tumor cell line was modified with MAA (P815-MAA) or left unmodified (P815) and 10(6) irradiated cells were injected into DBA/2 mice once a week for 5 weeks. Serum was collected and tested for antibody responses to P815 cells and the MAA epitope. Immunization of MAA adducted P815 cells into syngeneic DBA/2 mice induced a strong antibody response to the MAA epitope as determined by ELISA on Alb and MAA-Alb (508 microg/ml and 1092 microg/ml, respectively). In addition, antibody to unmodified P815 cells was detected by fluorescent technique. Mice immunized with P815 cells or PBS showed little or no reactivity to the MAA epitope or P815 cells. Studies to assess IL-12 stimulation showed that peritoneal macrophages from P815 and PBS immunized animals produced modest amounts of IL-12 (20 and 35 pg/ml) when stimulated with Alb or MAA-Alb. However, macrophage from P815-MAA immunized mice responded to soluble MAA adduct (142 pg/ml). Finally, in tumor survival studies the mean survival was 14.25 days in PBS treated mice; 15.75 days with P815 immunized mice and 18.25 days with P815-MAA immunized mice. Therefore, these data strongly suggest that antibody responses are induced by P815 cells modified with MAA adducts. This may be a possible tool to begin looking at how alcohol metabolites potentially modify cells and/or cellular components making them recognizable to the immune system as foreign. It is thought that these studies define a model system that will be useful in assessing antibody and potentially T-cell responses to cells that are modified by MAA.

CYP2E1 and oxidative liver injury by alcohol

Ethanol-induced oxidative stress seems to play a major role in mechanisms by which ethanol causes liver injury. Many pathways have been suggested to contribute to the ability of ethanol to induce a state of oxidative stress. One central pathway seems to be the induction of cytochrome P450 2E1 (CYP2E1) by ethanol. CYP2E1 metabolizes and activates many toxicological substrates, including ethanol, to more reactive, toxic products. Levels of CYP2E1 are elevated under a variety of physiological and pathophysiological conditions and after acute and chronic alcohol treatment. CYP2E1 is also an effective generator of reactive oxygen species such as the superoxide anion radical and hydrogen peroxide and, in the presence of iron catalysts, produces powerful oxidants such as the hydroxyl radical. This review article summarizes some of the biochemical and toxicological properties of CYP2E1 and briefly describes the use of cell lines developed to constitutively express CYP2E1 and CYP2E1 knockout mice in assessing the actions of CYP2E1. Possible therapeutic implications for treatment of alcoholic liver injury by inhibition of CYP2E1 or CYP2E1-dependent oxidative stress will be discussed, followed by some future directions which may help us to understand the actions of CYP2E1 and its role in alcoholic liver injury.

Metabolization of Porphyrinogenic Agents in Brain: Involvement of the Phase I Drug Metabolizing System. A Comparative Study in Liver and Kidney

(1) We evaluated the involvement of brain mitochondrial and microsomal cytochrome P-450 in the metabolization of known porphyrinogenic agents, with the aim of improving the knowledge on the mechanism leading to porphyric neuropathy. We also compared the response in brain, liver and kidney. To this end, we determined mitochondrial and microsomal cytochrome P-450 levels and the activity of NADPH cytochrome P-450 reductase. (2) Animals were treated with known porphyrinogenic drugs such as volatile anaesthetics, allylisopropylacetamide, veronal, griseofulvin and ethanol or were starved during 24 h. Cytochrome P-450 levels and NADPH cytochrome P-450 reductase activity were measured in mitochondrial and microsomal fractions from the different tissues. (3) Some of the porphyrinogenic agents studied altered mitochondrial cytochrome P-450 brain but not microsomal cytochrome P-450. Oral griseofulvin induced an increase in mitochondrial cytochrome P-450 levels, while chronic Isoflurane produced a reduction on its levels, without alterations on microsomal cytochrome P-450. Allylisopropylacetamide diminished both mitochondrial and microsomal cytochrome P-450 brain levels; a similar pattern was detected in liver. Mitochondria cytochrome P-450 liver levels were only diminished after chronic Isoflurane administration. In kidney only mitochondrial cytochrome P-450 levels were modified by veronal; while in microsomes, only acute anaesthesia with Enflurane diminished cytochrome P-450 content. (4) Taking into account that delta-aminolevulinic acid would be responsible for porphyric neuropathy, we investigated the effect of acute and chronic delta-aminolevulinic acid administration. Acute delta-aminolevulinic acid administration reduced brain and liver cytochrome P-450 levels in both fractions; chronic delta-aminolevulinic acid administration diminished only liver mitochondrial cytochrome P-450. (5) Brain NADPH cytochrome P-450 reductase activity in animals receiving allylisopropylacetamide, dietary griseofulvin and delta-aminolevulinic acid showed a similar profile as that for total cytochrome P-450 levels. The same response was observed for the hepatic enzyme. (6) Results here reported revealed differential tissue responses against the xenobiotics assayed and give evidence on the participation of extrahepatic tissues in porphyrinogenic drug metabolization. These studies have demonstrated the presence of the integral Phase I drug metabolizing system in the brain, thus, total cytochrome P-450 and associated monooxygenases in brain microsomes and mitochondria would be taken into account when considering the xenobiotic metabolizing capability of this organ.

CYP2E1-dependent oxidative stress and toxicity: role in ethanol-induced liver injury

Ethanol-induced oxidative stress plays a major role in the mechanisms by which ethanol causes liver injury. Many pathways contribute to how ethanol induces a state of oxidative stress. One central pathway appears to be the induction, by ethanol, of the CYP2E1 form of cytochrome P450 enzymes. CYP2E1 is of interest because it metabolises and activates many toxicological substrates, including ethanol, to more reactive products. Levels of CYP2E1 are elevated under a variety of physiological and pathophysiological conditions. CYP2E1 is an effective generator of reactive oxygen species. This review summarises some of the biochemical and toxicological properties of CYP2E1, and briefly describes the use of HepG2 cell lines in assessing the actions of CYP2E1. Future directions, which may help to better understand the actions of CYP2E1 and its role in alcoholic liver injury, are suggested.

Thiophene is Toxic to Cerebellar Granule Cells in Culture After Bioactivation by Rat Liver Enzymes

Several compounds that are not neurotoxic by themselves can cause toxic effects in vivo after enzymatic bioactivation. Thiophene is an industrial solvent known to produce degeneration primarily of the granule cells in the cerebellum when administered to animals in vivo. The mechanism for thiophene toxicity is not known, although it has been suggested that thiophene metabolism may lead to formation of oxidative intermediates that could function as the ultimate toxicants. In the present work we have used rat cerebellar granule cells (CGCs) in culture combined with rat liver postmitochondrial (S9) fraction as a source of biotransformation enzymes to test the toxicity of thiophene in vitro. The results demonstrate that thiophene is toxic to rat cerebellar granule cells in culture only after biotransformation. Furthermore, the toxic effects were reduced by cytochrome P450 (CYP) inhibitors and by scavengers of reactive molecules (alpha-tocopherol, reduced glutathione, and phenyl-N-tert-butylnitrone). These findings support the hypothesis that thiophene requires metabolism to produce the ultimate toxicant, and that the cytochrome P450 enzyme system is involved in the metabolism.

Pheromone anosmia in a scarab beetle induced by in vivo inhibition of a pheromone-degrading enzyme

Previous biochemical evidence suggests that a cytochrome P450 specific to male antennae of the pale-brown chafer, Phyllopertha diversa, has evolved as a pheromone-degrading enzyme. By using a bioinformatics approach, we have now cloned three P450 cDNAs: CYP4AW1, CYP4AW2, and CYP6AT1. RT-PCR indicated that CYP4AW2 is expressed in all tissues examined, that CYP6AT1 is antennae-rich, and that CYP4AW1 is antennae-specific. Both tissue specificity and electrophysiological studies strongly support that CYP4AW1 in P. diversa is a pheromone-degrading enzyme involved in pheromone inactivation. Highly sensitive, pheromone-specific olfactory receptor neurons in male antennae were completely desensitized by direct application of metyrapone into the sensillar lymph. When tested in the same or different individuals, the metyrapone treatment had no effect on olfactory receptor neurons tuned to the plant volatile (Z)-3-hexenyl acetate, which might be inactivated by an esterase. Metyrapone treatment did not affect pheromone reception in the Japanese beetle, Popillia japonica, in the scarab beetle, Anomala octiescostata, or in the Oriental beetle, Exomala orientalis. Metyrapone-induced anosmia was restricted to the pheromone detectors in P. diversa, which became insensitive to physiological concentrations of pheromones for a few minutes. As opposed to previous trials, the specificity of the inhibitor and pheromone system led to unambiguous evidence for the role of pheromone-degrading enzymes in the fast inactivation of pheromones.

Drug-induced liver injury

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

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

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

Immune-mediated drug-induced liver disease

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

New autoantibodies and autoantigens in autoimmune hepatitis

The molecular characterization of the autoreactivities associated with autoimmune liver disease will improve their diagnosis and enhance understanding of their pathogenic mechanisms. Surprisingly, little is known about the nature of the major autoreactivities associated with type 1 AIH, including homogeneous ANA and antibodies to microfilaments [3]. Type 1 AIH is, however, the prototype of autoimmune liver disease [103].

Mitochondrial targeted cytochrome P450 2E1 (P450 MT5) contains an intact N terminus and requires mitochondrial specific electron transfer proteins for activity

Hepatic mitochondria contain an inducible cytochrome P450, referred to as P450 MT5, which cross-reacts with antibodies to microsomal cytochrome P450 2E1. In the present study, we purified, partially sequenced, and determined enzymatic properties of the rat liver mitochondrial form. The mitochondrial cytochrome P450 2E1 was purified from pyrazole-induced rat livers using a combination of hydrophobic and ion-exchange chromatography. Mass spectrometry analysis of tryptic fragments of the purified protein further ascertained its identity. N-terminal sequencing of the purified protein showed that its N terminus is identical to that of the microsomal cytochrome P450 2E1. In reconstitution experiments, the mitochondrial cytochrome P450 2E1 displayed the same catalytic activity as the microsomal counterpart, although the activity of the mitochondrial enzyme was supported exclusively by adrenodoxin and adrenodoxin reductase. Mass spectrometry analysis of tryptic fragments and also immunoblot analysis of proteins with anti-serine phosphate antibody demonstrated that the mitochondrial cytochrome P450 2E1 is phosphorylated at a higher level compared with the microsomal counterpart. A different conformational state of the mitochondrial targeted cytochrome P450 2E1 (P450 MT5) is likely to be responsible for its observed preference for adrenodoxin and adrenodoxin reductase electron transfer proteins.

Cytotoxic effect of an anti-liver monoclonal autoantibody obtained after neonatal thymectomy in mice

A monoclonal autoantibody, LSA-1, against murine liver antigen was obtained by fusing spleen cells from a neonatally thymectomized BALB/c mouse with SP2/0 murine myeloma cells. The LSA-1 isotype was IgG2b and kappa. LSA-1 was specific to the liver, especially, to a liver-specific membrane lipoprotein (LSP) fraction. By Western blotting analysis, LSA-1 mainly detected a 100 kDa protein of LSP fraction. LSA-1 stained cytoplasm of the cryostat sections of liver in immunohistochemical analysis. Furthermore, the antigen recognized with LSA-1 was highly expressed on the surface of a murine hepatoma cell line, MH134, slightly on a murine normal liver cell line, C1469, and on freshly prepared hepatocytes, but not on spleen cells. LSA-1 had a cytotoxic activity on liver cell lines in the presence of a complement in vitro. Furthermore, injection of LSA-1 into mice-induced liver injury. These results suggest that anti-liver autoantibody plays an important role in the induction of autoimmune hepatitis. Accordingly, this antibody will be a useful tool for the analysis of the pathogenesis of autoimmune hepatitis.

Human microsomal epoxide hydrolase is the target of germander-induced autoantibodies on the surface of human hepatocytes

Germander, a plant used in folk medicine, caused an epidemic of cytolytic hepatitis in France. In about half of these patients, a rechallenge caused early recurrence, suggesting an immunoallergic type of hepatitis. Teucrin A (TA) was found responsible for the hepatotoxicity via metabolic activation by CYP3A. In this study, we describe the presence of anti-microsomal epoxide hydrolase (EH) autoantibodies in the sera of patients who drank germander teas for a long period of time. By Western blotting and immunocytochemistry, human microsomal EH was shown to be present in purified plasma membranes of both human hepatocytes and transformed spheroplasts and to be exposed on the cell surface where affinity-purified germander autoantibodies recognized it as their autoantigen. Immunoprecipitation of EH activity by germander-induced autoantibodies confirmed this finding. These autoantibodies were not immunoinhibitory. The plasma membrane-located EH was catalytically competent and may act as target for reactive metabolites from TA. To test this hypothesis CYP3A4 and EH were expressed with human cytochrome P450 reductase and cytochrome b(5) in a "humanized" yeast strain. In the absence of EH only one metabolite was formed. In the presence of EH, two additional metabolites were formed, and a time-dependent inactivation of EH was detected, suggesting that a reactive oxide derived from TA could alkylate the enzyme and trigger an immune response. Antibodies were found to recognize TA-alkylated EH. Recognition of EH present at the surface of human hepatocytes could suggest an (auto)antibody participation in an immune cell destruction.

Molecular basis for the transport of cytochrome P450 2E1 to the plasma membrane

Endoplasmic reticulum-resident cytochrome P450 enzymes that face the cytosol are present on the plasma membrane of hepatocytes, but the molecular origin for their transport to this compartment has until now remained unknown. The molecular basis for the transport of rat ethanol-inducible cytochrome P450 2E1 (CYP2E1) to the plasma membrane was investigated by transfection of several different mutant cDNAs into mouse H2.35 hepatoma cells. Two NH(2)-terminal CYP2E1 mutants were constructed: N(++)2E1, which carried two positive charges in the NH(2) terminus, and 2C-2E1, in which the transmembrane domain of CYP2E1 was replaced with that of CYP2C1, which was previously described to cause retention of CYP2C1 in the endoplasmic reticulum, as well as CYP2E1 COOH-terminally tagged with the vesicular stomatitis virus G protein (VSV-G) epitope (2E1-VSV-G). Immunofluorescent microscopy and cell surface biotinylation experiments revealed that all CYP2E1 variants were present on the extracellular side of the plasma membrane. The VSV-G epitope on CYP2E1 was detected on the outside of the plasma membrane using VSV-G-specific antibodies, indicating that the large COOH-terminal part of CYP2E1 is indeed exposed on the outside of the plasma membrane. The relative levels of CYP2E1, 2C-2E1, and 2E1-VSV-G on the cell surface were found to be about 2% of total cellular enzyme, whereas twice this amount of N(++)2E1 was recovered at the cell surface. Protease protection experiments performed on microsomes isolated from cDNA transfected cells revealed that a small fraction of CYP2E1 and all variant proteins was found to be located in the lumen of the endoplasmic reticulum (type II orientation), whereas the majority of the proteins were in the expected cytosolic or type I orientation. It is concluded that the NH(2)-terminal transmembrane domain of CYP2E1 plays a critical role in directing the protein to the cell surface and that topological inversion of a small fraction of CYP2E1 in the endoplasmic reticulum directs the protein to the plasma membrane.

Auto-antibodies in hepatitis C

Hepatitis C virus (HCV) has been implicated in the development of a variety of autoimmune phenomena, some of which are well documented and include a panel of auto-antibodies shared with autoimmune hepatitis (AIH). Anti-nuclear (ANA) and smooth muscle (SMA) antibodies (markers of AIH type 1 [AIH-1]), have been demonstrated in 9-38% and 5-91% of cases respectively, whereas anti-liver/kidney microsomal type 1 (anti-LKM-1) and anti-liver cytosol type 1 antibodies (anti-LC1) (markers of AIH type 2 [AIH-2]), are definitely rarer, especially in adults. The presence of these auto-reactivities in chronic hepatitis C generates clinical overlaps and dilemmas in the correct classification and treatment of such patients. The immunopathological characterization of the auto-antibodies, anti-nuclear and smooth muscle antibodies in particular, combined with internationally defined criteria for the diagnosis of AIH is helpful in this clinical process. Thyroid auto-antibodies and cryoprecitable rheumatoid factors are also commonly detected in hepatitis C, while the occurrence of other auto-antibodies still awaits confirmation.

Autoantibodies against cytochromes P-4502E1 and P-4503A in alcoholics

Autoantibodies against soluble liver enzymes have been reported among alcoholics, but the targets of self-reactivity toward membrane proteins of the liver have not been characterized. Previously, among alcoholics, we found antibodies against ethanol-derived radical protein adducts that are dependent on cytochrome P-4502E1 (CYP2E1) for their formation. To further investigate autoantibodies against cytochrome P-450s during alcohol abuse, sera of rats chronically treated with ethanol in the total enteral nutrition model and sera from alcoholics with or without alcohol liver disease and from control subjects were analyzed by enzyme-linked immunosorbent assay and Western blotting for the presence of IgG against rat and human CYP2E1, rat CYP3A1, and human CYP3A4. A time-dependent appearance of IgG against rat CYP3A1 and CYP2E1 was evident during chronic ethanol feeding of rats. Anti-CYP2E1 reactivity showed positive correlation with the levels of hepatic CYP2E1 and was inhibited by the CYP2E1 transcriptional inhibitor chlormethiazole. Screening of the human sera by enzyme-linked immunosorbent assay revealed reactivity against CYP3A4 and CYP2E1 in about 20 to 30% and 10 to 20% of the alcoholic sera, respectively. No difference were noted between sera from alcoholics with or without hepatitis C virus infection, and only very little reactivity was seen in sera from control subjects. Western blotting analysis revealed anti-human CYP2E1 reactivity in 8 of 85 alcoholic sera and 3 of 58 control sera, whereas anti-CYP3A4 reactivity was detected in 18 of 85 alcoholic sera and 4 of 58 control sera, which were different from the sera reactive with CYP2E1. Immunoblot reactivity of CYP3A4-positive alcoholic sera was found against glutathione-S-transferase fusion proteins containing truncated forms of CYP3A4, and such sera were also able to immunoprecipitate in vitro translated CYP3A4. Seven of eight sera showed reactivity toward domains C-terminal of position Ser281, and 1 of 8 sera recognized autoepitopes within the region Thr207-Ser281. These findings indicate that alcoholics develop autoantibodies against CYP2E1 and CYP3A4 that the CYP3A4 C-terminal domain is a target for the autoantibody reactions among a subset of alcoholics. The novel finding of CYP3A4 autoantibodies and their significant expression among alcoholics warrants further investigation. Attention should be given to immune toxicity associated with CYP3A4 autoantibodies and cases of alcohol abuse that are accompanied by exposure to drugs and substances that are CYP3A substrates.

Drug-induced immunotoxicity

Immune-related drug responses are one of the most common sources of idiosyncratic toxicity. A number of organs may be the target of such reactions; however, this review concentrates mostly on the liver. Drug-induced hepatitis is generally divided into two categories: acute hepatitis in which the drug or a metabolite destroys a vital target in the cell; immunoallergic hepatitis in which the drug triggers an adverse immune response directed against the liver. Their clinical features are: a) low frequency; b) dose independence; c) typical immune system manifestations such as fever, eosinophilia; d) delay between the initiation of treatment and onset of the disease; e) a shortened delay upon rechallenge; and f) occasional presence of autoantibodies in the serum of patients. Such signs have been found in cases of hepatitis triggered by drugs such as halothane, tienilic acid, dihydralazine and anticonvulsants. They will be taken as examples to demonstrate the recent progress made in determining the mechanisms responsible for the disease. The following mechanisms have been postulated: 1) the drug is first metabolized into a reactive metabolite which binds to the enzyme that generated it; 2) this produces a neoantigen which, once presented to the immune system, might trigger an immune response characterized by 3) the production of antibodies recognizing both the native and/or the modified protein; 4) rechallenge leads to increased neoantigen production, a situation in which the presence of antibodies may induce cytolysis. Toxicity is related to the nature and amount of neoantigen and also to other factors such as the individual immune system. An effort should be made to better understand the precise mechanisms underlying this kind of disease and thereby identify the drugs at risk; and also the neoantigen processes necessary for their introduction into the immune system. An animal model would be useful in this regard.

Yeast expressed cytochrome P450 2D6 (CYP2D6) exposed on the external face of plasma membrane is functionally competent

CYP2D6, a xenobiotic metabolizing cytochrome P450 (P450), was found to be present in significant amount on the outer face of cell plasma membrane in addition to the regular microsomal location. Present work demonstrates that this external P450 is catalytically competent and that activity is supported by NADPH-P450 reductase present on the inner face of plasma membrane. Purified plasma membranes from yeast expressing CYP2D6 sustained NADPH- and cumene hydroperoxide-dependent dextromethorphan demethylation and NADPH-cytochrome c activity confirming previous observations in human hepatocytes. CYP2D6 found on the outside of plasma membrane (by differential immuno-inhibition and acidic shift assays on transformed spheroplasts) was catalytically competent at the cell surface for NADPH-supported activities. Anti-yeast P450-reductase antibodies inhibited neither CYP2D6 nor P450-reductase activities upon incubation with intact spheroplasts. In contrast, both activities were inhibited on isolated plasma membrane fragments. This highly suggested a cytosolic-orientation of the plasma membrane P450-reductase. This finding was confirmed by immunostaining in confocal microscopy. Finally, gene deletion of P450-reductase caused a complete loss of plasma membrane NADPH-supported CYP2D6 activity, which suggests that the reductase participates to some degree in the transmembrane electron transfer chain. This work illustrates that the outside-exposed plasma membrane CYP2D6 is active and may play an important metabolic role.

Cytochrome P450: a novel system modulating Ca2+ channels and contraction in mammalian heart cells

1. Cytochrome P450 (P450) is a ubiquitous enzyme system that catalyses oxidative reactions of numerous endogenous and exogenous compounds. The modulatory effects of P450 on the L-type Ca2+ current (ICa), intracellular free Ca2+ signals and cell shortening were assessed in adult rat single ventricular myocytes. 2. Bath administration of the imidazole antimycotics, clotrimazole, econazole and miconazole, which are potent P450 inhibitors, significantly suppressed cardiac ICa. While the Ca2+ channel antagonist nifedipine blocked ICa within 30 s, clotrimazole-induced suppression of ICa required 5.1 +/- 0.4 min (n = 14) to reach a steady low level. The suppression of ICa was dose dependent and recovered after washout of clotrimazole. Intracellular dialysis with the P450 antibody anti-rat CYP1A2 also significantly reduced cardiac ICa. 3. Additional administration of the beta-adrenergic agonist isoprenaline (1 microM) or the membrane-permeable 8-bromo-cAMP (2 mM) completely reversed the suppressant effects of clotrimazole and NaCN on ICa. In addition, intracellular dialysis with 2 mM cAMP abolished the P450 inhibitor-induced suppression of ICa. Phosphorylation of the channel with hydrolysis-resistant ATPgammaS prevented the suppressant effect of clotrimazole on ICa. Furthermore, dephosphorylation of the Ca2+ channel with intracellular dialysis with phosphatase types I and II reduced ICa by 85 +/- 3 % and abolished clotrimazole-induced suppression of ICa. 4. Extracellular administration of the phospholipase A2 inhibitors mepacrine and 4-bromophenacyl bromide significantly suppressed ICa. 5. Clotrimazole, econazole, miconazole and CN- also significantly inhibited intracellular free Ca2+ signals and cell shortening in rat single ventricular myocytes. 6. Intracellular cAMP content was significantly reduced in isolated ventricular myocytes incubated with clotrimazole or CN-. Extracellular administration of 11, 12-epoxyeicosatrienoic acid, one of the P450-mediated metabolites of arachidonic acid, enhanced ICa and intracellular cAMP content. The epoxyeicosatrienoic acid also restored the amplitude of the reduced ICa in P450 antibody-dialysed myocytes. 7. The present data suggest that cytochrome P450 modulates cardiac ICa and cell contraction, and the modulation may result from changes in intracellular levels of cAMP by P450- mediated metabolites of arachidonic acid.

Topology inversion of CYP2D6 in the endoplasmic reticulum is not required for plasma membrane transport

The presence of CYP2D6 at the surface of isolated rat and human hepatocytes and its recognition by autoantibodies were reported recently. We wondered whether the unexpected outside orientation at the plasma membrane could be related to topological inversion (luminal-oriented form) of cytochrome P450 in the endoplasmic reticulum. To examine the potential role of cDNA polymorphism, a CYP2D6 variant carrying three positive charges at the amino terminus (2D6ext) was constructed and expressed in yeast. Immunoblotting, flow cytometry, and electron microscopy showed that wild-type CYP2D6 expressed in yeast was present on the outer face of the cell plasma membrane in addition to the regular microsomal location. This location reproduces the hepatocyte situation. 2D6ext expressed in yeast and COS7 cells seemed to be partially N-glycosylated and was located at the plasma membrane surface. Nevertheless, the glycosylated form was not enriched in the plasma membranes compared with microsomes. The relationship between CYP2D6 and 2D6ext topologies and catalytic competence was tested. Cumene hydroperoxide-dependent dextromethorphan demethylation was performed on microsomal vesicles after combined proteolysis and immunoinhibition experiments. CYP2D6 activity was completely abolished, whereas the glycosylated and luminal-oriented fraction of 2D6ext remained active. This suggests that a luminal-oriented glycosylated form is not involved in cytochrome P450 transport to the plasma membrane. Yeast thus reproduces the unusual CYP2D6 plasma membrane location and orientation, which do not require sequence alteration, glycosylation, or even an inverted endoluminal orientation.

Recent developments in autoimmune liver diseases

Several diseases are regarded as autoimmune liver diseases. Apart from the cholestatic liver diseases, primary biliary cirrhosis, primary sclerosing cholangitis, these include autoimmune hepatitis, hepatitis as part of the autoimmune polyendocrine syndrome type 1 (APS-1) and particular overlap syndromes such as autoimmune cholangitis (also called antimitochondrial antibody negative primary biliary cirrhosis [PBC]), overlap syndrome chronic active hepatitis (CAH)/PBC and the overlap syndrome primary sclerosing hepatitis (PSC)/CAH. In addition, auto-antibodies may be observed during the course of chronic viral hepatitis, in particular chronic hepatitis C and D. Finally, a small number of drug-induced liver diseases is immune mediated. The following article will review our recent progress in the field of autoimmune hepatitis including APS-1 and autoimmunity in viral hepatitis and immune-mediated drug-induced liver disease.

Binding proteins for cyclic and linear oligopeptides in plasma membranes and the cytosol of rat hepatocytes

Using a cyclolinopeptide A analogue, the hydrophobic cyclic peptide c(-Ala-Lys-Pro-Phe-Phe-Ala-Lys-Pro-Phe-Phe-), termed CDP (cyclodecapeptide), as ligand in affinity chromatography, hepatocellular peptide binding proteins were isolated from the integral part of plasma membranes and the cytosol. The sequence of the isolated protein with MW of 50 kDa from the integral part of the plasma membrane fraction was identical to cytochrome P450 II C13 and cytochrome P450 II C22, whereas the sequence of the 54 kDa protein was identical to 3-hydroxyandrogen-UDP-glucuronosyltransferase. These proteins have also been described as binding proteins for bile acids. As shown in earlier studies, bile acids and CDP also compete for uptake into hepatocytes. In the cytosol, a further known bile acid binding protein, the glutathione-S-transferase (G-S-T) subunit Yb1, was isolated and sequenced as binding protein for CDP and also for a further cyclopeptide, the somatostatin analogue OO8, and a linear peptide with renin-inhibiting activity, EMD 55068. As shown in uptake studies using isolated basolateral plasma membrane vesicles, G-S-T was able to increase the uptake of EMD 51921, a linear peptide with renin-inhibiting potency, into the vesicles when the latter were preloaded with G-S-T. The binding of the substrate to the outside of the preloaded vesicles was not different than binding to unloaded vesicles. The maximal transport rate of the carrier-mediated/facilitated diffusion and the rate of permeation, however, were doubled in the presence of G-S-T, pointing to the involvement of intracellular binding proteins such as G-S-T in the unloading of the carrier protein and in the reduction of the free substrate concentration.

Immunotoxicology of the liver: adverse reactions to drugs

Liver is a frequent target for drug-induced hepatitis. They can be classified in two categories: the hepatitis in which the drug or a metabolite reach a vital target in the cell and the hepatitis in which the drug triggers an adverse immune response directed against the liver. We will discuss essentially this second kind of disease. They have key clinical features such as the low frequency, the dose independence, the delay between the beginning of drug intake and the triggering of the disease, the shortening of the delay upon rechallenge and very often the presence of autoantibodies in the serum of the patients. Such signs were found in hepatitis triggered by drugs such as halothane, tienilic acid, dihydralazine, anticonvulsants. They will be taken as examples to show the recent progress in the understanding of the mechanisms leading to the disease. It has been postulated that the drug is metabolised into a reactive metabolite binding to the enzyme which generated it; therefore the neoantigen might trigger an immune response characterised by the production of antibodies recognising the native and or the modified protein. Most of these steps were proven in the cases of halothane, tienilic acid and dihydralazine. Several points seem important in the development of the disease; the equilibrium between toxication and detoxication pathways, the nature and amount of neoantigen, the individual immune response. However, many points remain unclear: for instance, the reason for the very low frequency of this kind of disease; the precise mechanism of the adverse immune response; the risk factors for developing such adverse reactions. Efforts should be made to better understand the mechanisms of this kind of disease: for instance, an animal model, tests to identify drugs at risk for such reactions, the role of these drugs in the processing of P450s and the processing of the neoantigens for their presentation to the immune system.

Antigenic targets in tienilic acid hepatitis. Both cytochrome P450 2C11 and 2C11-tienilic acid adducts are transported to the plasma membrane of rat hepatocytes and recognized by human sera

Patients with tienilic acid hepatitis exhibit autoantibodies that recognize unalkylated cytochrome P450 2C9 in humans but recognize 2C11 in rats. Our aim was to determine whether the immune reaction is also directed against neoantigens. Rats were treated with tienilic acid and hepatocytes were isolated. Immunoprecipitation, immunoblotting, and flow cytometry experiments were performed with an anti-tienilic acid or an anti-cytochrome P450 2C11 antibody. Cytochrome P450 2C11 was the main microsomal or plasma membrane protein that was alkylated by tienilic acid. Inhibitors of vesicular transport decreased flow cytometric recognition of both unalkylated and tienilic acid-alkylated cytochrome P450 2C11 on the plasma membrane of cultured hepatocytes. Tienilic acid hepatitis sera that were preadsorbed on microsomes from untreated rats (to remove autoantibodies), poorly recognized untreated hepatocytes in flow cytometry experiments, but better recognized tienilic acid-treated hepatocytes. This recognition was decreased by adsorption with tienilic acid or by preexposure to the anti-tienilic acid or the anti-cytochrome P450 2C11 antibody. We conclude that cytochrome P450 2C11 is alkylated by tienilic acid and follows a vesicular route to the plasma membrane. Tienilic acid hepatitis sera contain antibodies against this tienilic acid adduct, in addition to the previously described anticytochrome P450 autoantibodies.

Autoimmune hepatitis. Definition--classification--histopathology--immunopathogenesis

Autoimmune hepatitis (AIH) is a distinct form of acute and chronic inflammatory liver disease in which immune reactions against host antigens are found to be the major pathological mechanism. If left untreated it carries an unfavourable prognosis, and the diagnosis should be made as soon as possible. The diagnostic approach has been greatly facilitated by the establishment of a panel of marker autoantibodies, which do not define distinct therapeutic groups of AIH, but do allow a subgrouping based on differences in patient populations, some clinical features and prognosis. The characterization of organ-specific components of the liver cell surface as targets of cellular and humoral autoimmune reactions give new insights into the pathogenesis of the disease, even though the primary event triggering the disease remains to be defined. The most important disease-promoting factor seems to be a genetically determined background for autoimmunity. Without this different environmental factors, including viruses, toxins, cytokines and drugs, are only able to induce transient autoimmune phenomena and not autoimmune disease. The histopathology of AIH is in keeping with the present pathogenetic concept. Although there is no pathognomonic feature distinguishing this type of hepatitis from virus-induced forms, some distinct morphological lesions are regarded as characteristic. Clinical research on AIH has benefited greatly from observations of experimental AIH in mice. Recognition of the critical role of autoreactive T-lymphocytes in the pathogenesis and the observation of spontaneous recovery from AIH in the animal model associated with antigen-specific and antigen-non-specific T-cell suppression have made basic contributions to our improved understanding of the natural course of AIH in humans.

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.

The functional expression of sodium-dependent bile acid transport in Madin-Darby canine kidney cells transfected with the cDNA for microsomal epoxide hydrolase

Previous studies have suggested that the enzyme microsomal epoxide hydrolase (mEH) is able to mediate sodium-dependent transport of bile acids such as taurocholate into hepatocytes (von Dippe, P., Amoui, M., Alves, C., and Levy, D.(1993) Am. J. Physiol. 264, G528-G534). In order to characterize directly the putative transport properties of the enzyme, a pCB6 vector containing the cDNA for this protein (pCB6-mEH) was transfected into Madin-Darby canine kidney (MDCK) cells, and stable transformants were isolated that could express mEH at levels comparable with the levels expressed in hepatocytes. Sodium-dependent transport of taurocholate was shown to be dependent on the expression of mEH and to be inhibited by the bile acid transport inhibitor 4,4'-diisothiocyanostilbene-2,2'disulfonic acid (DIDS), as well as by other bile acids. Kinetic analysis of this system indicated a Km of 26.3 microM and a Vmax of 117 pmol/mg protein/min. The Km value is essentially the same as that observed in intact hepatocytes. The transfected MDCK cells also exhibited sodium-dependent transport of cholate at levels 150% of taurocholate in contrast to hepatocytes where cholate transport is only 30% of taurocholate levels, suggesting that total hepatocyte bile acid transport is a function of multiple transport systems with different substrate specificities, where mEH preferentially transports cholate. This hypothesis is further supported by the observation that a monoclonal antibody that partially protects (26%) taurocholate transport from inhibition by DIDS in hepatocytes provides almost complete protection (88%) from DIDS inhibition of hepatocyte cholate transport, suggesting that taurocholate is also taken up by an alternative system not recognized by this antibody. Additional support for this concept is provided by the observation that the taurocholate transport system is almost completely protected (92%) from DIDS inhibition by this antibody in MDCK cells that express mEH as the only bile acid transporter. These results demonstrate that mEH is expressed on the surface of hepatocytes as well as on transfected MDCK cells and is able to mediate sodium-dependent transport of taurocholate and cholate.

Detection of aromatase cytochrome P450, 17 alpha-hydroxylase cytochrome P450 and NADPH:P450 reductase on the surface of cells in which they are expressed

Using the membrane impermeant probe NHS-LC-biotin, we show in this report that a fraction of aromatase P450 (P450arom), the enzyme that catalyzes estrogen biosynthesis, is present at the surface of cells in which it is expressed, either endogenously or as a consequence of transfection. The same findings were obtained for a truncated form of P450arom lacking the putative membrane-spanning region, thus suggesting the presence of other membrane-spanning region(s) within its structure. P450arom is not unique in this regard as we find that a fraction of 17 alpha-hydroxylase P450 as well as NADPH:P450 reductase also are present at the cell surface. It is therefore possible that a number of microsomal P450s are expressed at the cell surface in this fashion.

Cytochrome P450 enzymes and UDP-glucuronosyltransferases as hepatocellular autoantigens

Cytochromes P450 and UDP-Glucuronosyltransferases (UGT) are targets of microsomal autoantibodies in liver and kidney (LKM). LKM autoantibodies are observed in autoimmune hepatitis, in some patients with viral hepatitis, drug-induced hepatitis and autoimmune hepatitis as disease component of the autoimmune polyglandular syndrome type 1 (APS-1). In autoimmune hepatitis LKM antibodies are markers of autoimmune hepatitis type 2. The major target of LKM-1 antibodies is cytochrome P450 2D6; a second less frequent target was the described UGTs of family 1. In autoimmune hepatitis LKM-1 autoantibodies are usually directed against small linear epitopes. LKM autoantibodies are also associated with infection with hepatitis viruses C and D. In hepatitis C about 1-2% of patients develop LKM-1 autoantibodies. About 60% of these autoantibodies are conformation dependent. The presence of LKM autoantibodies in hepatitis C may be associated with an increased risk in interferon treatment. LKM-3 autoantibodies are found in about 8% of patients with hepatitis D and are directed against conformational epitopes. Patients treated with certain drugs may develop drug induced hepatitis. In hepatitis induced by tienilic acid, tienilic acid is activated by and covalently bound to cytochrome P450 2C9. Activation of the immune system results in the formation of autoantibodies against cytochrome P450 2C9 (LKM-2) and infiltration of the liver with immune cells. A similar mechanism has been described for dihydralazine induced hepatitis, where autoantibodies are directed against P450 1A2 (LM). Autoantibodies directed against cytochrome P450 1A2 also are found in patients suffering from hepatitis as a disease component of APS-1.

Cytochrome P4502B follows a vesicular route to the plasma membrane in cultured rat hepatocytes

BACKGROUND/AIMS

Autoantibodies against cytochrome P450 are found in some forms of autoimmune hepatitis. Cytochrome P450 is synthesized and mainly located in the endoplasmic reticulum but may also be expressed on the plasma membrane of hepatocytes. Vesicles migrate from the endoplasmic reticulum to the Golgi apparatus and then to the plasma membrane along microtubules. We determined the route followed by cytochrome P4502B to reach the plasma membrane.

METHODS

Rat hepatocytes were cultured for 2 hours after plating with various inhibitors of cellular trafficking. Detached, uncut, nonpermeabilized hepatocytes were then exposed to a monoclonal antibody specific for cytochrome P4502B and studied by flow cytometry and confocal microscopy.

RESULTS

The plasma membrane expression of cytochrome P4502B was markedly decreased after 2 hours of culture with cycloheximide (an inhibitor of protein synthesis), caffeine at 20 degrees C (conditions that decrease vesicular transport from the endoplasmic reticulum to the Golgi apparatus), brefeldin A (which redistributes Golgi components back to the endoplasmic reticulum), monensin (an inhibitor of Golgi functions), and colchicine, vinblastine, or nocodazole (three microtubule inhibitors).

CONCLUSIONS

Part of cytochrome P4502B follows a microtubule-dependent vesicular route from the endoplasmic reticulum to the plasma membrane in cultured rat hepatocytes.

Detection of antibodies against proteins modified by hydroxyethyl free radicals in patients with alcoholic cirrhosis

BACKGROUND/AIMS

We have previously shown that hydroxyethyl free radicals produced during cytochrome P4502E1-mediated oxidation of ethanol covalently bind to microsomal proteins. The present study examined whether alkylation of proteins by hydroxyethyl radicals induces an immunologic response in alcoholic patients.

METHODS

A microplate enzyme-linked immunosorbent assay was developed using as antigen human serum albumin or bovine fibrinogen reacted with chemically produced hydroxyethyl radicals.

RESULTS

This assay showed that the sera of alcoholic cirrhotics contained both immunoglobulin (Ig) Gs and IgAs that recognized proteins modified by hydroxyethyl radicals, whereas practically no reaction was observed in the sera of healthy controls or cirrhotics without evidence of alcohol abuse. The reactivity of the sera from alcoholic patients was not influenced by the protein to which hydroxyethyl radicals were bound. The sera of alcoholic cirrhotics also contained antibodies directed against acetaldehyde-modified albumin. However, the reaction of alcoholic sera with hydroxyethyl radical epitopes was not inhibited by increasing concentrations of acetaldehyde-modified albumin produced under either reducing or nonreducing conditions.

CONCLUSIONS

The results indicate that a new group of antigens that do not cross-react with antibodies against acetaldehyde-derived epitopes is formed by the alkylation of protein by hydroxyethyl radicals and is involved in the development of immunologic reactions in alcoholic patients.

Binding proteins for cyclosomatostatins and bile acids in basolateral plasma membranes of rat liver

The bile acids cholate and taurocholate on the one hand and the cyclopeptide c(Phe-Thr-Lys-Trp-Phe-D-Pro) (008), an analog of somatostatin with retro sequence, on the other hand, display mutually competitive transport inhibition into isolated rat hepatocytes. This indicates a common transport system for bile acids and cyclosomatostatins in sinusoidal rat liver plasma membranes. In order to identify and isolate common binding and/or transport proteins for bile acids and the cyclopeptides by affinity chromatography, the bile acid derivative 4'-amino-7-benzamidotaurocholate (ABATC) and the cyclosomatostatin-analog 008 were attached to a gel matrix. Two methods were used to prepare integral membrane proteins: (1) alkaline EDTA extraction and (2) Triton X-114 phase separation. Octyl glycoside solubilized, alkaline EDTA-extracted integral basolateral membrane proteins with apparent molecular masses of 52 and 48 kDa bound specifically to the ABATC affinity matrix. Two-phase Triton X-114 separated integral membrane proteins of the same molecular masses bound specifically to the cyclosomatostatin ligand. The 48 kDa ABATC and 008 binding protein was shown to be present in the basolateral plasma membrane fraction and in the microsomal fraction. The isolated 52 kDa ABATC binding protein was localized only in basolateral plasma membranes and could not be found in isolated microsomes.

Autoimmune hepatitis and hepatitis C virus infection

The identification of the hepatitis C virus (HCV) and the availability of serological tests for the identification of its infection has deeply changed our view of autoimmune hepatitis. In fact, we have learned that autoantibodies such as anti-nuclear, anti-smooth muscle and anti-liver kidney microsomes, cannot be considered specific any longer for the diagnosis of autoimmune hepatitis, since they are frequently found in association with HCV. The new clinical entity characterized by the association of autoantibodies with signs of HCV infection is presently under evaluation. This, in order to understand what is the prevalent pathogenetic mechanism, viral or autoimmune, operating in these patients and to chose the best treatment regimen.