Mitochondrial dysfunction in paracetamol hepatotoxicity: in vitro studies in isolated mouse hepatocytes

Uptake and Biotransformation of the Tire Rubber-derived Contaminants 6-PPD and 6-PPD Quinone in the Zebrafish Embryo (Danio rerio)

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6-PPD) is a widely used antioxidant in tire rubber known to enter the aquatic environment via road runoff. The associated transformation product (TP) 6-PPD quinone (6-PPDQ) causes extreme acute toxicity in some fish species (e.g., coho salmon). To interpret the species-specific toxicity, information about biotransformation products of 6-PPDQ would be relevant. This study investigated toxicokinetics of 6-PPD and 6-PPDQ in the zebrafish embryo (ZFE) model. Over 96 h of exposure, 6-PPD and 6-PPDQ accumulated in the ZFE with concentration factors ranging from 140 to 2500 for 6-PPD and 70 to 220 for 6-PPDQ. A total of 22 TPs of 6-PPD and 12 TPs of 6-PPDQ were tentatively identified using liquid chromatography coupled to high-resolution mass spectrometry. After 96 h of exposure to 6-PPD, the TPs of 6-PPD comprised 47% of the total peak area (TPA), with 4-hydroxydiphenylamine being the most prominent in the ZFE. Upon 6-PPDQ exposure, >95% of 6-PPDQ taken up in the ZFE was biotransformed, with 6-PPDQ + O + glucuronide dominating (>80% of the TPA). Among other TPs of 6-PPD, a reactive N-phenyl-p-benzoquinone imine was found. The knowledge of TPs of 6-PPD and 6-PPDQ from this study may support biotransformation studies in other organisms.

The Hepatic Mitochondrial Pyruvate Carrier as a Regulator of Systemic Metabolism and a Therapeutic Target for Treating Metabolic Disease

Pyruvate sits at an important metabolic crossroads of intermediary metabolism. As a product of glycolysis in the cytosol, it must be transported into the mitochondrial matrix for the energy stored in this nutrient to be fully harnessed to generate ATP or to become the building block of new biomolecules. Given the requirement for mitochondrial import, it is not surprising that the mitochondrial pyruvate carrier (MPC) has emerged as a target for therapeutic intervention in a variety of diseases characterized by altered mitochondrial and intermediary metabolism. In this review, we focus on the role of the MPC and related metabolic pathways in the liver in regulating hepatic and systemic energy metabolism and summarize the current state of targeting this pathway to treat diseases of the liver. Available evidence suggests that inhibiting the MPC in hepatocytes and other cells of the liver produces a variety of beneficial effects for treating type 2 diabetes and nonalcoholic steatohepatitis. We also highlight areas where our understanding is incomplete regarding the pleiotropic effects of MPC inhibition.

Use of in vitro metabolomics in NRK cells to help predicting nephrotoxicity and differentiating the MoA of nephrotoxicants

We describe a strategy using an in vitro metabolomics assay with tubular rat NRK-52E cells to investigate the Modes of Action (MoAs) of nephrotoxic compounds. Chemicals were selected according to their MoAs based on literature information: acetaminophen, 4-aminophenol and S-(trichlorovinyl-)L-cysteine (TCVC), (covalent protein binding); gentamycin, vancomycin, polymycin B and CdCl₂ (lysosomal overload) and tenofovir and cidofovir (mitochondrial DNA-interaction). After treatment and harvesting of the cells, intracellular endogenous metabolites were quantified relative to vehicle control. Metabolite patterns were evaluated in a purely data-driven pattern generation process excluding published information. This strategy confirmed the assignment of the chemicals to the respective MoA except for TCVC and CdCl₂. Finally, TCVC was defined as unidentified and CdCl₂ was reclassified to the MoA "covalent protein binding". Hierarchical cluster analysis of 58 distinct metabolites from the patterns enabled a clear visual separation of chemicals in each MoA. The assay reproducibility was very good and metabolic responses were consistent. These results support the use of metabolome analysis in NRK-52E cells as a suitable tool for understanding and investigating the MoA of nephrotoxicants. This assay could enable the early identification of nephrotoxic compounds and finally reduce animal testing.

Divergent effects of glutathione depletion on isocitrate dehydrogenase 1 and the pentose phosphate pathway in hamster liver

The liver regenerates NADPH via multiple pathways to maintain redox balance and reductive biosynthesis. The pentose phosphate pathway (PPP) contributes to hepatic lipogenesis by supplying NADPH, and it is thought to play a major role in response to oxidative stress. This study determined the significance of the PPP and related NADPH-regenerating enzymes in the liver under oxidative stress. Fasted hamsters received acetaminophen (400 mg/kg) to deplete glutathione in the liver and [U-13 C3 ]glycerol to measure the PPP activity by analysis of 13 C distribution in plasma glucose. Blood and liver were harvested to assess NADPH-producing enzymes, antioxidant defense, PPP, and other relevant biochemical processes. Acetaminophen caused glutathione depletion and decreased activities of glutathione peroxidase and catalase in the liver, but it did not change triglyceride synthesis. Although the PPP is potentially an abundant source of NADPH, its activity was decreased and the expression of glucose 6-phosphate dehydrogenase remained unchanged after acetaminophen treatment. The effects of acetaminophen on other NADPH-producing enzymes were complex. Isocitrate dehydrogenase 1 was overexpressed, both isocitrate dehydrogenase 2 and malic enzyme 1 were underexpressed, and methylenetetrahydrofolate dehydrogenase 1 remained unchanged. In summary, isocitrate dehydrogenase 1 was most sensitive to glutathione depletion caused by acetaminophen, but glucose 6-phosphate dehydrogenase, the regulatory enzyme of PPP, was not.

Mitochondrial functions of THP-1 monocytes following the exposure to selected natural compounds

The immune system is an important target of various xenobiotics, which may lead to severe adverse effects including immunosuppression or inappropriate immunostimulation. Mitochondrial toxicity is one possibility by which xenobiotics exert their toxic effects in cells or organs. In this study, we investigated the impact of three natural compounds, cyclosporine A (CsA), deoxynivalenol (DON) and cannabidiol (CBD) on mitochondrial functions in the THP-1 monocytic cell line. The cells were exposed for 24h to two different concentrations (IC₁₀ and IC₅₀ determined by MTT) of each compound. The cells showed concentration-dependent elevated intracellular reactive oxygen species (iROS) and induction of apoptosis (except DON) in response to the three test compounds. Mitochondrial functions were characterized by using bioenergetics profiling experiments. In THP-1 monocytes, the IC₅₀ of CsA decreased basal and maximal respiration as well as ATP production with an impact on spare capacity indicating a mitochondrial dysfunction. Similar reaction patterns were observed following CBD exposure. The basal respiration level and ATP-production decreased in the THP-1 cells exposed to the IC₅₀ of DON with no major impact on mitochondrial function. In conclusion, impaired mitochondrial function was accompanied by elevated iROS and apoptosis level in a monocytic cell line exposed to CsA and CBD. Mitochondrial dysfunction may be one explanation for the cytotoxicity of CBD and CsA also in other in immune cells.

Acetaminophen cytotoxicity is ameliorated in a human liver organotypic co-culture model

Organotypic liver culture models for hepatotoxicity studies that mimic in vivo hepatic functionality could help facilitate improved strategies for early safety risk assessment during drug development. Interspecies differences in drug sensitivity and mechanistic profiles, low predictive capacity, and limitations of conventional monocultures of human hepatocytes, with high attrition rates remain major challenges. Herein, we show stable, cell-type specific phenotype/cellular polarity with differentiated functionality in human hepatocyte-like C3A cells (enhanced CYP3A4 activity/albumin synthesis) when in co-culture with human vascular endothelial cells (HUVECs), thus demonstrating biocompatibility and relevance for evaluating drug metabolism and toxicity. In agreement with in vivo studies, acetaminophen (APAP) toxicity was most profound in HUVEC mono-cultures; whilst in C3A:HUVEC co-culture, cells were less susceptible to the toxic effects of APAP, including parameters of oxidative stress and ATP depletion, altered redox homeostasis, and impaired respiration. This resistance to APAP is also observed in a primary human hepatocyte (PHH) based co-culture model, suggesting bidirectional communication/stabilization between different cell types. This simple and easy-to-implement human co-culture model may represent a sustainable and physiologically-relevant alternative cell system to PHHs, complementary to animal testing, for initial hepatotoxicity screening or mechanistic studies of candidate compounds differentially targeting hepatocytes and endothelial cells.

A self-regulating cell culture analog device to mimic animal and human toxicological responses

The overall goal of this project is the development of a new methodology for translating advances in molecular level understanding of toxicological responses into a predictive tool for dose response in whole animals and humans exposed to single compounds or mixtures of compounds. The methodology incorporates a mechanistic cellular level model into a PBPK (physiologically based pharmacokinetic) model which simultaneously guides the development of an in vitro cell culture analog (CCA) to the PBPK. Where the PBPK specifies an organ, (e.g., liver) the in vitro or CCA system contains a compartment with the appropriate cell or cell population (e.g., hepatocytes for the liver). The CCA has significant advantages over other in vitro systems and PBPK systems used independently for evaluating metabolic responses to drugs or potentially toxic chemicals where the exchange of metabolites between organs is likely to be important. The CCA system is superior to a PBPK because an a priori description of complete metabolism is not required and secondary, unexpected interactions can be detected. The CCA system, unlike other in vitro systems, gives a dynamic response that realistically simulates in vivo interactions between organs. Furthermore, the CCA allows dosing on the same basis as animal tests (e.g., milligrams per kilogram of body mass equivalent). Because the construction of a CCA is guided by a PBPK, this approach allows extrapolation to low doses and across species, including extrapolation to humans. We have constructed a prototype system and have conducted proof-of-concept experiments using naphthalene as a test chemical. These experiments clearly demonstrate the ability to generate a reactive metabolite in one compartment and detect its effects (on LDH release and glutathione depletion) in a second compartment. However, this prototype device would be expensive to replicate and requires nearly constant supervision from a trained investigator. For this concept to replace animals an inexpensive, self-regulating device is needed. An initial design to accomplish this goal is described as well as the corresponding model using naphthalene as a test compound. (c) 1996 John Wiley & Sons, Inc.

Assessment of viability of hepatocytes in suspension using the MTT assay

The viability and state of proliferation of cells in culture is conveniently assessed using MTT, which is metabolically reduced to stain functionally intact cells. The hypothesis was tested that this assay can also be used in the quantitation of effects of toxicants on hepatocytes in suspension. Hepatocytes isolated from BALB/c mice were incubated without or with menadione, rotenone, N-methylformamide or paracetamol. Cellular damage was measured by either MTT assay or release into the medium of lactate dehydrogenase (LDH). Results obtained with the two tests were compatible in the case of toxicity inflicted by menadione or N-methylformamide. Rotenone decreased cell viability as indicated by the MTT assay immediately after addition of the agent, whereas measurement of LDH release did not detect this rapid toxic effect. The MTT assay detected paracetamol-induced damage within the first hour of exposure, but this was not detected by the LDH assay until 3 hr had elapsed.

Apical membrane rupture and backward bile flooding in acetaminophen-induced hepatocyte necrosis

Morphological changes of hepatocyte death have so far only been described on cells in culture or in tissue sections. Using a high-resolution and high-magnification multiphoton microscopic system, we recorded in living mice serial changes of acetaminophen (APAP)-induced hepatocyte necrosis in relevance to metabolism of a fluorogenic bile solute. Initial changes of hepatocyte injury included basal membrane disruption and loss of mitochondrial membrane potential. An overwhelming event of rupture at adjacent apical membrane resulting in flooding of bile into these hepatocytes might ensue. Belbs formed on basal membrane and then dislodged into the sinusoid circulation. Transmission electron microscopy disclosed a necrotic hepatocyte depicting well the changes after apical membrane rupture and bile flooding. Administration of the antidote N-acetylcysteine dramatically reduced the occurrence of apical membrane rupture. The present results demonstrated a hidden but critical step of apical membrane rupture leading to irreversible APAP-induced hepatocyte injury.

Understanding lactic acidosis in paracetamol (acetaminophen) poisoning

Paracetamol (acetaminophen) is one of the most commonly taken drugs in overdose in many areas of the world, and the most common cause of acute liver failure in both the UK and USA. Paracetamol poisoning can result in lactic acidosis in two different scenarios. First, early in the course of poisoning and before the onset of hepatotoxicity in patients with massive ingestion; a lactic acidosis is usually associated with coma. Experimental evidence from studies in whole animals, perfused liver slices and cell cultures has shown that the toxic metabolite of paracetamol, N-acetyl-p-benzo-quinone imine, inhibits electron transfer in the mitochondrial respiratory chain and thus inhibits aerobic respiration. This occurs only at very high concentrations of paracetamol, and precedes cellular injury by several hours. The second scenario in which lactic acidosis can occur is later in the course of paracetamol poisoning as a consequence of established liver failure. In these patients lactate is elevated primarily because of reduced hepatic clearance, but in shocked patients there may also be a contribution of peripheral anaerobic respiration because of tissue hypoperfusion. In patients admitted to a liver unit with paracetamol hepatotoxicity, the post-resuscitation arterial lactate concentration has been shown to be a strong predictor of mortality, and is included in the modified King's College criteria for consideration of liver transplantation. We would therefore recommend that post-resuscitation lactate is measured in all patients with a severe paracetamol overdose resulting in either reduced conscious level or hepatic failure.

Reactive nitrogen species in acetaminophen-induced mitochondrial damage and toxicity in mouse hepatocytes

Acetaminophen (APAP) toxicity in primary mouse hepatocytes occurs in two phases. The initial phase (0-2 h) occurs with metabolism to N-acetyl-p-benzoquinoneimine which depletes glutathione, and covalently binds to proteins, but little toxicity is observed. Subsequent washing of hepatocytes to remove APAP and reincubating in media alone (2-5 h) results in toxicity. We previously reported that the reincubation phase occurs with mitochondrial permeability transition (MPT) and increased oxidative stress (dichlorodihydrofluorescein fluorescence) (DCFH(2)). Since DCFH(2) may be oxidized by multiple oxidative mechanisms, we investigated the role of reactive nitrogen species (RNS) leading to 3-nitrotyrosine in proteins by ELISA and by immunoblots. Incubation of APAP with hepatocytes for 2 h did not result in toxicity or protein nitration; however, washing hepatocytes and reincubating in media alone (2-5 h) resulted in protein nitration which correlated with toxicity. Inclusion of the MPT inhibitor, cyclosporine A, in the reincubation media eliminated toxicity and protein nitration. The general nitric oxide synthase (NOS) inhibitor L-NMMA and the neuronal NOS (NOS1) inhibitor, 7-nitroindazole, added in the reincubation media decreased toxicity and protein nitration; however, neither the inducible NOS (NOS2) inhibitors L-NIL (N6-(1-iminoethyl)-L-lysine) nor SAIT (S-(2-aminoethyl)isothiourea) decreased protein nitration or toxicity. The RNS scavengers, N-acetylcysteine, and high concentrations of APAP, added in the reincubation phase decreased toxicity and protein nitration. 7-Nitroindazole and cyclosporine A inhibited the APAP-induced loss of mitochondrial membrane potential when added in the reincubation phase. The data indicate a role for RNS in APAP induced toxicity.

Anti-lipid peroxidation and protection of liver mitochondria against injuries by picroside II

AIM: To investigate the anti-lipid peroxidation and protection of liver mitochondria against injuries in mice with liver damage by picroside II.

METHODS: Three animal models of liver damage induced by carbon tetrachloride (CCl₄: 0.1 mL/10 g, ip), D-galactosamine (D-GalN: 500 mg/kg, ip) and acetaminophen (AP: 0.15 g/kg, ip) were respectively treated with various concentrations of picroside II (5, 10, 20 mg/kg, ig). Then we chose the continuously monitoring method (recommended by International Clinical Chemistry League) to analyze serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) values, Marland method to detect the activity of manganese-superoxide dismutase (SOD) in liver mitochondria, TBA colorimetry to determine the content of malonicdialdehyde (MDA) in liver tissue, DTNB method to evaluate the activity of glutathioneperoxidase (GSH-Px) and Lowry method to detect protein level in liver tissue. Meanwhile, effects of picroside II on the activity of ATPase and swelling extent of mitochondria in hepatocytes damaged by AP were also evaluated.

RESULTS: Picroside II could significantly prevent liver toxicity in the three models of liver damage. It decreased the high levels of ALT and AST in serum induced by the administration of CCl₄, D-GalN and AP, reduced the cellular damage of liver markedly, and appeared to be even more potent than the positive control drug of biphenyl dimethyl dicarboxylate pilules (DDB). In groups treated with different doses of picroside II, compared to the model group, the content of MDA in serum decreased evidently, whereas the content of SOD and GSH-Px increased in a dose-dependent manner, and the difference was statistically significant. Further, in the study of AP model, picroside II inhibited AP-induced liver toxicity in mice, enhanced the activity of ATPase, improved the swelling extent of mitochondria and helped to maintain a normal balance of energy metabolism.

CONCLUSION: Picroside II can evidently relieve hepatocyte injuries induced by CCl₄, D-GalN and AP, help scavenge free radicals, protect normal constructions of mitochondria membrane and enhance the activity of ATPase in mitochondria, thereby modulating the balance of liver energy metabolism, which might be part of the mechanisms of hepatoprotective effects of picroside II.

Mitochondrial permeability transition in acetaminophen-induced necrosis and apoptosis of cultured mouse hepatocytes

Acetaminophen overdose causes massive hepatic failure via mechanisms involving glutathione depletion, oxidative stress, and mitochondrial dysfunction. The ultimate target of acetaminophen causing cell death remains uncertain, and the role of apoptosis in acetaminophen-induced cell killing is still controversial. Our aim was to evaluate the mitochondrial permeability transition (MPT) as a key factor in acetaminophen-induced necrotic and apoptotic killing of primary cultured mouse hepatocytes. After administration of 10 mmol/L acetaminophen, necrotic killing increased to more than 49% and 74%, respectively, after 6 and 16 hours. MPT inhibitors, cyclosporin A (CsA), and NIM811 temporarily decreased necrotic killing after 6 hours to 26%, but cytoprotection was lost after 16 hours. Confocal microscopy revealed mitochondrial depolarization and inner membrane permeabilization approximately 4.5 hours after acetaminophen administration. CsA delayed these changes, indicative of the MPT, to approximately 11 hours after acetaminophen administration. Apoptosis indicated by nuclear changes, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling, and caspase-3 activation also increased after acetaminophen administration. Fructose (20 mmol/L, an adenosine triphosphate-generating glycolytic substrate) plus glycine (5 mmol/L, a membrane stabilizing amino acid) prevented nearly all necrotic cell killing but paradoxically increased apoptosis from 37% to 59% after 16 hours. In the presence of fructose plus glycine, CsA decreased apoptosis and delayed but did not prevent the MPT. In conclusion, after acetaminophen a CsA-sensitive MPT occurred after 3 to 6 hours followed by a CsA-insensitive MPT 9 to 16 hours after acetaminophen. The MPT then induces ATP depletion-dependent necrosis or caspase-dependent apoptosis as determined, in part, by ATP availability from glycolysis.

Gene expression profiling reveals multiple toxicity endpoints induced by hepatotoxicants

Microarray technology continues to gain increased acceptance in the drug development process, particularly at the stage of toxicology and safety assessment. In the current study, microarrays were used to investigate gene expression changes associated with hepatotoxicity, the most commonly reported clinical liability with pharmaceutical agents. Acetaminophen, methotrexate, methapyrilene, furan and phenytoin were used as benchmark compounds capable of inducing specific but different types of hepatotoxicity. The goal of the work was to define gene expression profiles capable of distinguishing the different subtypes of hepatotoxicity. Sprague-Dawley rats were orally dosed with acetaminophen (single dose, 4500 mg/kg for 6, 24 and 72 h), methotrexate (1mg/kg per day for 1, 7 and 14 days), methapyrilene (100mg/kg per day for 3 and 7 days), furan (40 mg/kg per day for 1, 3, 7 and 14 days) or phenytoin (300 mg/kg per day for 14 days). Hepatic gene expression was assessed using toxicology-specific gene arrays containing 684 target genes or expressed sequence tags (ESTs). Principal component analysis (PCA) of gene expression data was able to provide a clear distinction of each compound, suggesting that gene expression data can be used to discern different hepatotoxic agents and toxicity endpoints. Gene expression data were applied to the multiplicity-adjusted permutation test and significantly changed genes were categorized and correlated to hepatotoxic endpoints. Repression of enzymes involved in lipid oxidation (acyl-CoA dehydrogenase, medium chain, enoyl CoA hydratase, very long-chain acyl-CoA synthetase) were associated with microvesicular lipidosis. Likewise, subsets of genes associated with hepatotocellular necrosis, inflammation, hepatitis, bile duct hyperplasia and fibrosis have been identified. The current study illustrates that expression profiling can be used to: (1) distinguish different hepatotoxic endpoints; (2) predict the development of toxic endpoints; and (3) develop hypotheses regarding mechanisms of toxicity.

Acetaminophen-induced suppression of hepatic AdoMet synthetase activity is attenuated by prodrugs of L-cysteine

Administration of acetaminophen (ACP, 400 mg/kg, i.p.) to fasted, male Swiss-Webster mice caused a rapid 90% decrease in total hepatic glutathione (GSH) and a 58% decrease in mitochondrial GSH by 2 h post ACP. This was followed by a time-dependent decrease (72%) in hepatic AdoMet synthetase activity and rise in plasma ALT levels (>10000 U/l) at 24 h post ACP treatment. AdoMet synthetase activity was maintained at 82, 78 and 60% of controls, respectively, by the cysteine prodrugs PTCA, CySSME and NAC. Total hepatic and mitochondrial GSH levels were also protected from severe ACP-induced depletion by CySSME and MTCA. These results suggest that the maintenance of GSH homeostasis by cysteine prodrugs can protect mouse hepatic AdoMet synthetase, a sulfhydryl enzyme whose integrity is dependent on GSH, as well as the liver itself from the consequences of oxidative stress elicited by toxic metabolites of xenobiotics.

Paracetamol (acetaminophen)-induced toxicity: molecular and biochemical mechanisms, analogues and protective approaches

An overview is presented on the molecular aspects of toxicity due to paracetamol (acetaminophen) and structural analogues. The emphasis is on four main topics, that is, bioactivation, detoxication, chemoprevention, and chemoprotection. In addition, some pharmacological and clinical aspects are discussed briefly. A general introduction is presented on the biokinetics, biotransformation, and structural modification of paracetamol. Phase II biotransformation in relation to marked species differences and interorgan transport of metabolites are described in detail, as are bioactivation by cytochrome P450 and peroxidases, two important phase I enzyme families. Hepatotoxicity is described in depth, as it is the most frequent clinical observation after paracetamol-intoxication. In this context, covalent protein binding and oxidative stress are two important initial (Stage I) events highlighted. In addition, the more recently reported nuclear effects are discussed as well as secondary events (Stage II) that spread over the whole liver and may be relevant targets for clinical treatment. The second most frequent clinical observation, renal toxicity, is described with respect to the involvement of prostaglandin synthase, N-deacetylase, cytochrome P450 and glutathione S-transferase. Lastly, mechanism-based developments of chemoprotective agents and progress in the development of structural analogues with an improved therapeutic index are outlined.

Metabolites of acetaminophen trigger Ca2+ release from liver microsomes

Release of mitochondrial calcium is believed to play a key role in the toxicity of acetaminophen in biological systems. Elevated cytosolic Ca2+ may also result from activation of calcium releasing channels. The major metabolites of acetaminophen, benzoquinone imine and 1,4-benzoquinone, induced Ca2+ release in isolated rat liver microsomes. The 1,4-benzoquinone-induced release of calcium was suppressed by ryanodine and fully inhibited by reduced glutathione. Concentrations of 1,4-benzoquinone that induced Ca2+ release did not affect the activity of the microsomal Ca2+, Mg2+-APTase. The binding of [3H]ryanodine to liver microsomes, however, was significantly decreased by 1,4-benzoquinone, suggesting a direct interaction of this metabolite with the ryanodine-binding protein (ryanodine receptor). These results suggest that cellular Ca2+ levels may be elevated by acetaminophen by pathways involving, in part, activation of Ca2+ releasing channels such as the ryanodine receptor.

Paracetamol hepatotoxicity and microsomal function

The effect of paracetamol-induced hepatotoxicity in rats (650 mg/kg) on microsomal function was examined. Paracetamol treatment resulted in lowered Na(+),K(+)-ATPase activity in the microsomes with decrease in V(max) of the low affinity high V(max) component II. However, the temperature kinetics was not influenced significantly. The total phospholipid and cholesterol contents as well as lipid peroxidation in the microsomes were unchanged. However, content of acidic phospholipids: phosphatidylserine and phosphatidylinositol decreased by 50% with a reciprocal increase in the sphingomyelin content; the lysophosphoglyceride content increased by 12-fold. The microsomal membrane appeared to be more fluidized following paracetamol treatment. Paracetamol treatment also resulted in a significant reduction in the sulfhydryl groups content.

Inhibition of protein phosphatase activity and changes in protein phosphorylation following acetaminophen exposure in cultured mouse hepatocytes

Protein phosphorylation was determined in cultured mouse hepatocytes exposed to an hepatotoxic concentration of acetaminophen (APAP) for selected times up to 12 h. Cultures were radiolabled with 32P-orthophosphoric acid and the cell extracts were analyzed by 2D gel electrophoresis and autoradiography. APAP exposure selectively increased the phosphorylation state of proteins of molecular weight 22, 25, 28, and 59 kDa and decreased the phosphorylation of a 26-kDa protein. Evidence is presented that these changes (1) are dependent on cytochrome P-450 activation of APAP; (2) occur well before enzyme leakage in this in vitro model; (3) are not likely attributed to GSH depletion alone; (4) are in part mimicked by okadaic acid, calyculin A, and cantharidic acid, three structurally distinct inhibitors of protein phosphatases 1 and 2A; and (5) are paralleled by a decline in protein phosphatase activity. The physiological consequences of protein phosphatase inactivation could be significant in APAP overdose since these enzymes are involved in the dephosphorylation of regulatory proteins that control many cell functions. This study also provides the first evidence for disruption in signal transduction pathways as a response to or component of APAP-induced hepatic injury.

Identification of the hepatic protein targets of reactive metabolites of acetaminophen in vivo in mice using two-dimensional gel electrophoresis and mass spectrometry

Liver toxicity following an overdose of acetaminophen is frequently considered a model for drug-induced hepatotoxicity. Extensive studies over many years have established that such toxicity is well correlated with liver protein arylation by acetaminophen metabolites. Identification of protein targets for covalent modifications is a challenging but necessary step in understanding how covalent binding could lead to liver toxicity. Previous approaches suffered from technical limitations, and thus over the last 10 years heroic efforts were required to determine the identity of only a few target proteins. We present a new mass spectrometry-based strategy for identification of all target proteins that now provides a comprehensive survey of the suite of liver proteins modified. After administration of radiolabeled acetaminophen to mice, the proteins in the liver tissue lysate were separated by two-dimensional polyacrylamide gel electrophoresis. In-gel digestion of the radiolabeled gel spots gave a set of tryptic peptides, which were analyzed by matrix-assisted laser desorption ionization mass spectrometry. Interrogation of data bases based on experimentally determined molecular weights of peptides and product ion tags from postsource decay mass spectra was employed for the determination of the identities of modified liver proteins. Using this method, more than 20 new drug-labeled proteins have been identified.

Antioxidant enzyme status in biliary obstructed rats: effects of N-acetylcysteine

BACKGROUND/AIMS

N-acetylcysteine (NAC) is a modulator of thiol levels that protects against hepatotoxic agents. The aim of this study was to investigate whether NAC might improve hepatic antioxidant defenses in chronically biliary obstructed rats.

METHODS

Secondary biliary cirrhosis was induced by 28 days of bile-duct obstruction. Groups of control and cirrhotic animals received NAC (50 mumol .kg-1.d-1 i.m.) through the experimental period.

RESULTS

Bile-duct obstruction resulted in decreased liver glutathione concentrations. Dichlorofluorescein (DCF) and thiobarbituric acid reactive substances (TBARS) concentrations, measured as markers of production of reactive oxygen species and lipid peroxidation, respectively, were significantly increased. Microsomal and mitochondrial membrane fluidity and the activities of catalase, cytosolic and mitochondrial superoxide dismutase (SOD), glutathione S-transferase, and cytosolic and mitochondrial Se-dependent and Se-independent glutathione peroxidase (GPx) were significantly reduced. NAC corrected the reduction in glutathione concentration and partially prevented the increases in DCF and TBARS concentrations. In addition, NAC treatment resulted in significant preservation of membrane fluidity and of the activities of catalase, mitochondrial SOD and the different forms of GPx.

CONCLUSIONS

Our data indicate that NAC maintains antioxidant defenses in biliary obstructed rats. These effects of NAC suggest that it may be a useful agent to preserve liver function in patients with biliary obstruction.

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.

Protein targets of xenobiotic reactive intermediates

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

Effect of acetaminophen administration on hepatic glutathione compartmentation and mitochondrial energy metabolism in the rat

Changes in cell energy metabolism and mitochondrial dysfunction have been observed after acetaminophen administration. Because consumption of hepatic glutathione is closely related to acetaminophen toxicity, we investigated the kinetics of: 1. glutathione depletion in liver mitochondria and cytosol; 2. State 3 and 4 respiratory rates of succinate-supplemented mitochondria; 3. rate of ATP synthesis; 4. oligomycin-sensitive ATP hydrolase activity and passive proton conductivity of inside-out vesicles of the inner mitochondrial membrane; and 5. changes in hepatic and mitochondrial malondialdehyde in the rat after in vivo acetaminophen administration. Two hours after acetaminophen injection, hepatic glutathione decreased and malondialdehyde increased. In the same interval, an increase in both State 3 and 4 respiratory rates of succinate-supplemented mitochondria was observed. This was accompanied by a decrease in the rate of ATP synthesis and the P/O ratio and by an increase in the passive proton permeability of the inner mitochondrial membrane, which was insensitive to oligomycin. No significant change in oligomycin-sensitive ATP hydrolase activity was observed. Four hours after APAP injection, the respiratory rates, as well as the proton conductivity, decreased, the rate of ATP synthesis was restored, and the mitochondrial glutathione started to increase; the cytosolic levels of glutathione were still low and the cytosolic and mitochondrial levels of malondialdehyde remained high for 2 more hr. The concentrations of these indices were completely restored 24 hr postdosing. Our findings suggest that acetaminophen administration selectively depletes (within 2 hr) mitochondrial glutathione, and produces local toxicity by altering membrane permeability and decreasing the efficiency of oxidative phosphorylation. This renders mitochondria more susceptible to oxidative damage, especially during increased free radical production, as in the case of enhanced mitochondrial respiration in State 4. The concomitant restoration of mitochondrial respiration, oxidative phosphorylation, membrane permeability, and glutathione levels is consistent with the importance of the mitochondrial glutathione pool for the protection of the mitochondrial membrane against oxidative damage.

Is activation of lysosomal enzymes responsible for paracetamol-induced hepatotoxicity and nephrotoxicity?

Paracetamol overdose (300 mg kg(-1)) in mice resulted in a time-dependent increase in the liver weight; no change was seen for the kidney. The total acid phosphatase activities in the two tissues increased significantly 0.5 h after paracetamol overdose and remained elevated up to 3 h. Free as well as total cathepsin D activities increased significantly in both the tissues within 2-2.5 h of paracetamol treatment. Simultaneously tyrosine positive materials in the two tissues increased. RNAse II and DNAse II activities were low in liver and kidneys of the controls. Paracetamol treatment elevated both free and total RNAse II activity in the two tissues by 0.5 h. Maximum activity of DNAse II (free and total) was seen at 2.5 h after paracetamol administration. The results suggest that concerted action of cathepsin D, RNAse II and DNAse II may be responsible for paracetamol-induced hepatotoxicity and nephrotoxicity.

Glutamate dehydrogenase covalently binds to a reactive metabolite of acetaminophen

The mechanism of the hepatotoxicity of the analgesic acetaminophen is believed to be mediated by covalent binding to protein; however, critical targets which effect the toxicity are unknown. It has been shown that mitochondrial respiration in vivo is inhibited in mice as early as 1 h following a hepatotoxic dose of acetaminophen, and it is postulated that covalent binding to critical mitochondrial proteins may be important. A time course of mitochondrial proteins stained with anti-acetaminophen in an immunoblot detected two major adducts of 50 and 67 kDa as early as 30 min after a hepatotoxic dose of acetaminophen in mice. To further understand the role of covalent binding to mitochondrial proteins and acetaminophen hepatotoxicity, we have purified and identified a 50 kDa mitochondrial protein which becomes covalently bound to a reactive metabolite of acetaminophen. An N-terminal sequence of the 50 kDa adduct was 100% homologous with the deduced amino acid sequence of glutamate dehydrogenase. In addition, the purified protein was immunochemically reactive with rat liver anti-glutamate dehydrogenase. Enzyme activity of glutamate dehydrogenase was significantly decreased in mice 1 h following hepatotoxic treatment with acetaminophen. These data suggest that acetaminophen hepatotoxicity may in part be mediated by covalent binding to glutamate dehydrogenase.

Effects of acetaminophen on the ultrastructure of isolated rat hepatocytes

The effects of acetaminophen (AA) on the ultrastructure of isolated hepatocytes (IHC) of rat following incubation of IHC suspensions with AA were examined by electron microscopy. The effect of N-acetyl-p-benzoquinone imine (NAPQI), a putative toxic metabolite of AA, were also observed. IHC were prepared from livers obtained from phenobarbital-treated rats by collagenase perfusion method. With 5 and 20 mM AA, surface blebs mainly containing smooth endoplasmic reticulum (ER) occurred in IHC. Dilatation of Golgi apparatus, partial degranulation of rough ER and enlargement of mitochondria were also observed. The altered mitochondria showed a low electron-dense matrix with loss of mitochondrial granules. With 500 microM NAPQI, surface blebs containing various organelles occurred in IHC. Disorderly distributions of cytoplasmic organelles, mild dilatation of rough and smooth ER and cytoplasmic myeloid bodies were observed. The characteristic myeloid bodies were seemingly derived from degranulated rough ER.

Effects of N-acetylcysteine and dithiothreitol on glutathione and protein thiol replenishment during acetaminophen-induced toxicity in isolated mouse hepatocytes

Isolated mouse hepatocytes were incubated with 1.0 mM acetaminophen (AA) for 1.5 h to initiate glutathione (GSH) and protein thiol (PSH) depletion and cell injury. Cells were subsequently washed to remove non-covalently bound AA and resuspended in medium containing N-acetylcysteine (NAC, 2.0 mM) or dithiothreitol (DTT, 1.5 mM). The effects of these agents on the replenishment of GSH and total PSH content were related to the development of cytotoxicity. When cells exposed to AA were resuspended in medium containing NAC or DTT, both agents replenished GSH and total PSH content to levels observed in untreated cells but only DTT was able to attenuate cytotoxicity. Addition of the GSH synthesis inhibitor, buthionine sulfoximine (BSO, 1.0 mM, 1.5 h), to cells in incubation medium containing AA, enhanced GSH and total PSH depletion and potentiated cytotoxicity. Resuspension of these cells in medium containing NAC did not alter the potentiating effects of BSO; GSH and PSH levels were not replenished and no cytoprotective effects were observed. However, when cells exposed to AA and BSO were resuspended in medium containing DTT, PSH content was replenished but GSH levels were not restored. In addition, DTT was able to delay the development of cytotoxicity. It appears that DTT, unlike NAC, has a GSH-independent mechanism of PSH replenishment. These observations suggest that while replenishment of GSH and total PSH content does not result in cytoprotection, the regeneration of critical PSH by DTT may play an important role in the maintenance of proper cell structure and/or function.

Inhibition of mitochondrial respiration in vivo is an early event in acetaminophen-induced hepatotoxicity

Morphological changes in mitochondria are observed early in the course of acetaminophen (AA)-induced hepatotoxicity, and mitochondrial dysfunction has been observed both in vivo and in vitro following exposure to AA. This study examined the early effects of AA exposure in vivo on mitochondrial respiration and evaluated the effectiveness of N-acetyl-L-cysteine (NAC) in protecting against respiratory dysfunction. Mitochondria were isolated from the livers of fasted, male CD-1 mice 0, 0.5, 1, 1.5 or 2 h after administration of a hepatotoxic dose of AA (750 mg/kg). Glutamate- and succinate-supported mitochondrial respiration were subsequently assessed by polarographic measurement of state 3 (ADP-stimulated) and state 4 (resting) rates of oxygen consumption and determination of the corresponding respiratory control ratios (RCR: state 3/state 4) and ADP:O ratios. Hepatotoxicity was assessed histologically and by measuring plasma alanine aminotransferase (ALT) activity. The earliest sign of mitochondrial dysfunction observed in this study was a significant decrease in the ADP:O ratio for the oxidation of glutamate 1 h post-dosing. At 1.5 and 2 h post-dosing the RCRs for both glutamate- and succinate-supported respiration were significantly decreased. All of the respiratory parameters measured in this study were significantly decreased, with the exception of succinate-supported state 4 respiration which was significantly increased, 2 h after AA administration. Thus, inhibition of mitochondrial respiration preceded overt hepatic necrosis, indicated by an elevation of ALT activity, which was not observed until 3 and 4 h post-dosing. In addition, mitochondrial respiratory dysfunction correlated with morphological alterations.(ABSTRACT TRUNCATED AT 250 WORDS)

Protective effect of Artemisia scoparia extract against acetaminophen-induced hepatotoxicity

1. Hepatoprotective activity of hydro-methanolic extract of Artemisia scoparia (Compositae) was investigated against acetaminophen-induced hepatic damage. 2. Acetaminophen at a dose of 1 g/kg produced 100% mortality in mice while pretreatment of animals with plant extract (150 mg/kg) reduced the death rate to 20%. 3. Acetaminophen at a dose of 640 mg/kg produced liver damage in rats as manifested by the rise in serum levels of GOT and GPT to 1528 +/- 310 and 904 +/- 261 IU/l (n = 10) respectively, compared to respective control values of 80 +/- 11 and 38 +/- 09. 4. Pretreatment of rats with plant extract (150 mg/kg) lowered significantly the respective serum GOT and GPT levels to 85 +/- 11 and 23 +/- 06. 5. These results indicate that Artemisia scoparia contains hepatoprotective constituents and this study rationalizes the traditional use of this plant in hepatobiliary disorders.

The toxicological relevance of paracetamol-induced inhibition of hepatic respiration and ATP depletion

In order to elucidate the role of mitochondrial dysfunction in paracetamol-induced hepatotoxicity, the effects of paracetamol on the oxygen consumption and ATP content of the isolated perfused rat liver were correlated with parameters of hepatic viability and hepatotoxicity. Paracetamol at 5 g/L reduced the oxygen consumption of the livers by about 80% and hepatic ATP content by 96%. Hepatotoxicity was evident from the nearly complete interruption of bile secretion, a marked release of enzymes [glutamate-pyruvate transaminase (GPT), lactate dehydrogenase (LDH)] in the perfusate, a depletion of hepatic glutathione and an accumulation of calcium in the liver. Paracetamol-induced hepatotoxicity could be prevented completely by using livers from non-fasted rats as well as by addition of fructose to the perfusate of livers from fasted animals. Both treatments resulted in an increased energy supply from anaerobic glycolysis as evidenced by a large release of lactate and pyruvate into the perfusate, but did not inhibit paracetamol-induced decline of oxygen consumption. The decrease in hepatic oxygen consumption depended on the dose of paracetamol and occurred first at a concentration of 0.2 g/L (-10%). LDH and GPT release, on the other hand, was elevated at 2 and 5 g/L and calcium accumulation occurred at 5 g/L paracetamol only. Inhibition of mixed-function oxidases by dithiocarb did not prevent the decrease in oxygen consumption and the resulting hepatic injury induced by paracetamol. The oral administration of the high dose of 5 g/kg paracetamol in vivo to rats exerted strong hepatotoxicity but produced maximal serum levels of 800 mg/L paracetamol only and did not decrease hepatic oxygen consumption as measured in vitro. Our results show that in the isolated perfused rat liver in vitro, only high concentrations of paracetamol can produce "chemical hypoxia" by attacking mitochondria so as to cause hepatic injury. Such high concentrations of paracetamol are not attained in vivo, however. "Chemical hypoxia", thus, seems not to be relevant to the well-known hepatotoxic action of paracetamol.

The role of the nucleus and other compartments in toxic cell death produced by alkylating hepatotoxicants

Hepatocellular necrosis occurs under a wide range of pathological conditions. In most cases, toxic cell death takes place over a finite span of time, delayed from the point of initial injury and accompanied by homeostatic counterresponses that are varied and complex. The present strategies for discovering critical steps in cell death recognize that (1) different toxins produce similar morphologic changes that precede killing in widely varied cell types, and that (2) lethal events are likely to involve one or more compartmentalized functions that are common to most cells. Investigations of the plasma membrane, endoplasmic reticulum, cytoplasm, mitochondrion, and nucleus have greatly advanced our understanding of acute hepatocellular necrosis. This report examines each compartment but emphasizes molecular changes in the nucleus which may explain cell death caused by alkylating hepatotoxicants. Accumulating knowledge about two distinct modes of cell death, necrosis and apoptosis, indicates that loss of Ca2+ regulation and subsequent damage to DNA may be critical steps in lethal damage to liver cells by toxic chemicals.