Reversible and irreversible inhibition of hepatic mitochondrial respiration by acetaminophen and its toxic metabolite, N-acetyl-p-benzoquinoneimine (NAPQI)

Racing against time: leveraging preclinical models to understand pulmonary susceptibility to perinatal acetaminophen exposures

Over the past decade, clinicians have increasingly prescribed acetaminophen (APAP) for patients in the neonatal intensive care unit (NICU). Acetaminophen has been shown to reduce postoperative opiate burden, and may provide similar efficacy for closure of the patent ductus arteriosus (PDA) as nonsteroidal anti-inflammatory drugs (NSAIDs). Despite these potential benefits, APAP exposures have spread to increasingly less mature infants, a highly vulnerable population for whom robust pharmacokinetic and pharmacodynamic data for APAP are lacking. Concerningly, preclinical studies suggest that perinatal APAP exposures may result in unanticipated adverse effects that are unique to the developing lung. In this review, we discuss the clinical observations linking APAP exposures to adverse respiratory outcomes and the preclinical data demonstrating a developmental susceptibility to APAP-induced lung injury. We show how clinical observations linking perinatal APAP exposures to pulmonary injury have been taken to the bench to produce important insights into the potential mechanisms underlying these findings. We argue that the available data support a more cautious approach to APAP use in the NICU until large randomized controlled trials provide appropriate safety and efficacy data.

Pulmonary implications of acetaminophen exposures independent of hepatic toxicity

Both preclinical and clinical studies have demonstrated that exposures to acetaminophen (APAP) at levels that cause hepatic injury cause pulmonary injury as well. However, whether exposures that do not result in hepatic injury have acute pulmonary implications is unknown. Thus, we sought to determine how APAP exposures at levels that do not result in significant hepatic injury impact the mature lung. Adult male ICR mice (8-12 wk) were exposed to a dose of APAP known to cause hepatotoxicity in adult mice [280 mg/kg, intraperitoneal (ip)], as well as a lower dose previously reported to not cause hepatic injury (140 mg/kg, ip). We confirm that the lower dose exposures did not result in significant hepatic injury. However, like high dose, lower exposure resulted in increased cellular content of the bronchoalveolar lavage fluid and induced a proinflammatory pulmonary transcriptome. Both the lower and higher dose exposures resulted in measurable changes in lung morphometrics, with the lower dose exposure causing alveolar wall thinning. Using RNAScope, we were able to detect dose-dependent, APAP-induced pulmonary Cyp2e1 expression. Finally, using FLIM we determined that both APAP exposures resulted in acute pulmonary metabolic changes consistent with mitochondrial overload in lower doses and a shift to glycolysis at a high dose. Our findings demonstrate that APAP exposures that do not cause significant hepatic injury result in acute inflammatory, morphometric, and metabolic changes in the mature lung. These previously unreported findings may help explain the potential relationship between APAP exposures and pulmonary-related morbidity.

Bioactive properties: enhancement of hepatoprotective, antioxidant and DNA damage protective effects of golden grey mullet protein hydrolysates against paracetamol toxicity

This study was undertaken to examine the hepatoprotective, antioxidant, and DNA damage protective effects of protein hydrolysates from Liza aurata, against paracetamol overdose induced liver injury in Wistar rats. L. aurata protein hydrolysates (LAPHs) were mainly constituted by glutamic acid (Glu) and glutamine (Gln) and lysine (Lys). In addition, they contained high amounts of proline (Pro), leucine (Leu) and glycine (Gly). The molecular weight distribution of the hydrolysates was determined by size exclusion chromatography, which analyzed a representative hydrolysate type with a weight range of 3–20 kDa. The hepatoprotective effect of LAPHs against paracetamol liver toxicity was investigated by in vivo assay. Rats received LAPHs daily by gavage, for 45 days. Paracetamol was administrated to rats during the last five days of treatment by intraperitoneal injection. Paracetamol overdose induced marked liver damage in rats was noted by a significant increase in the activities of serum aspartate amino transferase (AST) and alanine amino transferase (ALT), and oxidative stress which was evident from decreased activity of the enzymatic antioxidants (superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx)), and level of glutathione (GSH), and increased concentration of lipid peroxidation products (MDA). Furthermore, paracetamol increased the DNA damage with liver histopathological changes. LAPH pretreatment significantly attenuated paracetamol-induced hepatotoxic effects, including oxidative damage, histopathological lesions, and apoptotic changes in the liver tissue. Interestingly, LAPHs restored the activities of antioxidant enzymes and the level of GSH, ameliorated histological and molecular aspects of liver cells. The present data suggest that paracetamol high-dose plays a crucial role in the oxidative damage and genotoxicity of the liver and therefore, some antioxidants such us LAPHs might be safe as hepatoprotectors. Altogether, our studies provide consistent evidence of the beneficial effect of LAPHs on animals treated with a toxic dose of paracetamol and might encourage clinical trials.

This study was undertaken to examine the hepatoprotective, antioxidant, and DNA damage protective effects of protein hydrolysates from Liza aurata, against paracetamol overdose induced liver injury in Wistar rats.

Development and validation of a dynamic outcome prediction model for paracetamol-induced acute liver failure: a cohort study

BACKGROUND

Early, accurate prediction of survival is central to management of patients with paracetamol-induced acute liver failure to identify those needing emergency liver transplantation. Current prognostic tools are confounded by recent improvements in outcome independent of emergency liver transplantation, and constrained by static binary outcome prediction. We aimed to develop a simple prognostic tool to reflect current outcomes and generate a dynamic updated estimation of risk of death.

METHODS

Patients with paracetamol-induced acute liver failure managed at intensive care units in the UK (London, Birmingham, and Edinburgh) and Denmark (Copenhagen) were studied. We developed prognostic models, excluding patients who underwent transplantation, using Cox proportional hazards in a derivation dataset, and tested in initial and recent external validation datasets. Mortality was estimated in patients who had emergency liver transplantation. Model discrimination was assessed using area under receiver operating characteristic curve (AUROC) and calibration by root mean square error (RMSE). Admission (day 1) variables of age, Glasgow coma scale, arterial pH and lactate, creatinine, international normalised ratio (INR), and cardiovascular failure were used to derive an initial predictive model, with a second (day 2) model including additional changes in INR and lactate.

FINDINGS

We developed and validated new high-performance statistical models to support decision making in patients with paracetamol-induced acute liver failure. Applied to the derivation dataset (n=350), the AUROC for 30-day survival was 0·92 (95% CI 0·88-0·96) using the day 1 model and 0·93 (0·88-0·97) using the day 2 model. In the initial validation dataset (n=150), the AUROC for 30-day survival was 0·89 (0·84-0·95) using the day 1 model and 0·90 (0·85-0·95) using the day 2 model. Assessment of calibration using RMSE in prediction of 30-day survival gave values of 0·1642 for the day 1 model and 0·0626 for the day 2 model. In the external validation dataset (n=412), the AUROC for 30-day survival was 0·91 (0·87-0·94) using the day 1 model and 0·91 (0·88-0·95) using the day 2 model, and assessment of calibration using RMSE gave values of 0·079 for the day 1 model and 0·107 for the day 2 model. Applied to patients who underwent emergency liver transplantation (n=116), median predicted 30-day survival was 51% (95% CI 33-85).

INTERPRETATION

The models developed here show very good discrimination and calibration, confirmed in independent datasets, and suggest that many patients undergoing transplantation based on existing criteria might have survived with medical management alone. The role and indications for emergency liver transplantation in paracetamol-induced acute liver failure require re-evaluation.

FUNDING

Foundation for Liver Research.

Targeting mitochondria with methylene blue protects mice against acetaminophen-induced liver injury

Acetaminophen (APAP) overdose is a frequent cause of drug-induced liver injury and the most frequent cause of acute liver failure in the Western world. Previous studies with mouse models have revealed that impairment of mitochondrial respiration is an early event in the pathogenesis, but the exact mechanisms have remained unclear, and therapeutic approaches to specifically target mitochondria have been insufficiently explored. Here, we found that the reactive oxidative metabolite of APAP, N-acetyl-p-benzoquinoneimine (NAPQI), caused the selective inhibition of mitochondrial complex II activity by >90% in both mouse hepatic mitochondria and yeast-derived complexes reconstituted into nanoscale model membranes, as well as the decrease of succinate-driven adenosine triphosphate (ATP) biosynthesis rates. Based on these findings, we hypothesized that methylene blue (MB), a mitochondria-permeant redox-active compound that can act as an alternative electron carrier, protects against APAP-induced hepatocyte injury. We found that MB (<3 µM) readily accepted electrons from NAPQI-altered, succinate-energized complex II and transferred them to cytochrome c, restoring ATP biosynthesis rates. In cultured mouse hepatocytes, MB prevented the mitochondrial permeability transition and loss of intracellular ATP without interfering with APAP bioactivation. In male C57BL/6J mice treated with APAP (450 mg/kg, intraperitoneally [IP]), MB (10 mg/kg, IP, administered 90 minutes post-APAP) protected against hepatotoxicity, whereas mice treated with APAP alone developed massive centrilobular necrosis and increased serum alanine aminotransferase activity. APAP treatment inhibited complex II activity ex vivo, but did not alter the protein expression levels of subunits SdhA or SdhC after 4 hours.

CONCLUSION

MB can effectively protect mice against APAP-induced liver injury by bypassing the NAPQI-altered mitochondrial complex II, thus alleviating the cellular energy crisis. Because MB is a clinically used drug, its potential application after APAP overdose in patients should be further explored.

Antidote removal during haemodialysis for massive acetaminophen overdose

CONTEXT

Haemodialysis is sometimes used for patients with massive acetaminophen overdose when signs of "mitochondrial paralysis" (lactic acidosis, altered mental status, hypothermia and hyperglycaemia) are present. The role of haemodialysis is debated, in part because the evidence base is weak and the endogenous clearance of acetaminophen is high. There is also concern because the antidote acetylcysteine is also dialyzable. We prospectively measured serum acetylcysteine concentrations during haemodialysis in three such cases.

CASE DETAILS

Three adults each presented comatose and acidemic 10 to ~18 h after ingesting > 1000mg/kg of acetaminophen. Two were hypothermic and hyperglycaemic. Serum lactate concentrations ranged from 7 mM to 12.5 mM. All three were intubated, and initial acetaminophen concentrations were as high as 5980 μM (900 μg/mL). An intravenous loading dose of 150 mg/kg acetylcysteine was initiated between 10.8 and ~18 h post ingestion, and additional doses were empirically administered during haemodialysis to compensate for possible antidote removal. A single run of 3-4 h of haemodialysis removed 10-20 g of acetaminophen (48-80% of remaining body burden), reduced serum acetaminophen concentrations by 56-84% (total clearance 3.4-7.8 mL/kg/min), accelerated native acetaminophen clearance (mean elimination half-life 580 min pre-dialysis, 120 min during and 340 min post-dialysis) and corrected acidemia. Extraction ratios of acetylcysteine across the dialysis circuit ranged from 73% to 87% (dialysance 3.0 to 5.3 mL/kg/min). All three patients recovered fully, and none developed coagulopathy or other signs of liver failure.

DISCUSSION

When massive acetaminophen ingestion is accompanied by coma and lactic acidosis, emergency haemodialysis can result in rapid biochemical improvement. As expected, haemodialysis more than doubles the clearance of both acetaminophen and acetylcysteine. Because acetylcysteine dosing is largely empirical, we recommend doubling the dose during haemodialysis, with an additional half-load when dialysis exceeds 6 h.

Lactic acidosis: recognition, kinetics, and associated prognosis

Lactic acidosis is a common condition encountered by critical care providers. Elevated lactate and decreased lactate clearance are important for prognostication. Not all lactate in the intensive care unit is due to tissue hypoxia or ischemia and other sources should be evaluated. Lactate, in and of itself, is unlikely to be harmful and is a preferred fuel for many cells. Treatment of lactic acidosis continues to be aimed the underlying source.

Acetaminophen-induced cytotoxicity on human normal liver L-02 cells and the protection of antioxidants

In vitro cell models, which can partially mimic in vivo responses, offer potentially sensitive tools for toxicological assessment. The objective of this study was to explore the possible mechanisms of acetaminophen (AP)-induced toxicity in human normal liver L-02 cells. The expression of the CYP2E1 enzyme, which is reported to transform AP to its toxic metabolites, was higher in L-02 than in Hep3B cells. Further cell viability and reduced glutathione (GSH) depletion after AP treatment were examined. After exposure to AP for 24 h, cell viability decreased in a concentration-dependent manner. Concentration-dependent GSH depletion was also observed after AP treatment for 48 h, indicating oxidative stress had occurred in L-02 cells. The effects of D, L-buthionine-(S, R)-sulfoximine (BSO), an inhibitor of GSH biosynthesis, and N-acetylcysteine (NAC), a precursor of GSH synthesis, on the cytotoxicity induced by AP were also investigated. BSO aggravated the cytotoxicity induced by AP while NAC ameliorated such cell death. Further results showed that 10 mM AP caused cell apoptosis after 48 h treatment based on the DNA fragmentation assay and western blot of caspase-3 activation, respectively. In addition, the protective effects of various well-known antioxidants against AP-induced hepatotoxicity were observed. Taken together, these results indicate that oxidative stress and cellular apoptosis are involved in AP-induced toxicity in human normal liver L-02 cells, and this cell line is a suitable in vitro cell model for AP hepatotoxicity study.

Mechanisms of pathogenesis in drug hepatotoxicity putting the stress on mitochondria

Mitochondria play key roles in aerobic life and in cell death. Thus, interference of normal mitochondrial function impairs cellular energy and lipid metabolism and leads to the unleashing of mediators of cell death. The role of mitochondria in cell death due to drug hepatotoxicity has been receiving renewed attention and it is therefore timely to assess the current status of this area.

Massive acetaminophen ingestion with early metabolic acidosis and coma: treatment with IV NAC and continuous venovenous hemodiafiltration

CONTEXT

We report the extraction of acetaminophen by continuous venovenous hemodiafiltration (CVVHD) during treatment of an acute ingestion of 200 g with a peak recorded serum acetaminophen level of 1,614 mg/L (10,652 micromol/L).

CASE DETAILS

The patient presented with early onset of coma, metabolic acidosis, and hypotension in the absence of significant hepatic injury. In addition to N-acetylcysteine (NAC) therapy, CVVHD was performed to manage the acid-base disturbance. Flow rate, effluent volume, and serum and effluent drug concentrations were obtained at hourly intervals. During 16 h of CVVHD the acetaminophen level dropped from 1,212 to 247 mg/L.

DISCUSSION

The average clearance of acetaminophen by CVVHD was 2.53 L/h, with removal of 24 g of acetaminophen over 16 h. As NAC is effective in preventing hepatic injury after acute acetaminophen overdose, the role of dialysis or CVVHD is limited.

Early anion gap metabolic acidosis in acetaminophen overdose

PURPOSE

The study aimed to determine the incidence and clinical significance of early high (>15 mEq/L) anion gap metabolic acidosis in acetaminophen (APAP) overdose.

METHODS

A retrospective review of a cohort of 74 patients presenting within 24 hours of APAP overdose was conducted.

RESULTS

Early high anion gap metabolic acidosis was present in 41% of patients on admission and persisted for 1.5 ± 0.1 days. The anion gap was associated with an elevated lactate level (4.5 ± 1 mmol/L) (r(2) = 0.66, P < .05), which persisted for 1 day. The lactate level increased in proportion to the APAP concentration (r(2) = 0.75, P < .05). Patients with increased anion gap had a higher incidence of confusion (48% vs 3%; P < .001) and lethargy (39% vs 6%; P = .003). Early high anion gap metabolic acidosis was found in the absence of shock or liver failure. All patients were treated with N-acetylcysteine and, despite the early high anion gap metabolic acidosis, none developed hepatic failure or hypoglycemia.

CONCLUSION

Early high anion gap metabolic acidosis in patients with APAP overdose is self-limited and does not predict clinical or laboratory outcomes. Persistent or late metabolic acidosis in the absence of liver failure is not likely due to APAP and should prompt a search for other causes of metabolic acidosis. Finally, APAP overdose should be considered in patients presenting to the emergency department with altered mental status, as this is a treatable condition when detected early.

Silymarin modulates doxorubicin-induced oxidative stress, Bcl-xL and p53 expression while preventing apoptotic and necrotic cell death in the liver

The emergence of silymarin (SMN) as a natural remedy for liver diseases, coupled with its entry into NIH clinical trial, signifies its hepatoprotective potential. SMN is noted for its ability to interfere with apoptotic signaling while acting as an antioxidant. This in vivo study was designed to explore the hepatotoxic potential of Doxorubicin (Dox), the well-known cardiotoxin, and in particular whether pre-exposures to SMN can prevent hepatotoxicity by reducing Dox-induced free radical mediated oxidative stress, by modulating expression of apoptotic signaling proteins like Bcl-xL, and by minimizing liver cell death occurring by apoptosis or necrosis. Groups of male ICR mice included Control, Dox alone, SMN alone, and Dox with SMN pre/co-treatment. Control and Dox groups received saline i.p. for 14 days. SMN was administered p.o. for 14 days at 16 mg/kg/day. An approximate LD(50) dose of Dox, 60 mg/kg, was administered i.p. on day 12 to animals receiving saline or SMN. Animals were euthanized 48 h later. Dox alone induced frank liver injury (>50-fold increase in serum ALT) and oxidative stress (>20-fold increase in malondialdehyde [MDA]), as well as direct damage to DNA (>15-fold increase in DNA fragmentation). Coincident genomic damage and oxidative stress influenced genomic stability, reflected in increased PARP activity and p53 expression. Decreases in Bcl-xL protein coupled with enhanced accumulation of cytochrome c in the cytosol accompanied elevated indexes of apoptotic and necrotic cell death. Significantly, SMN exposure reduced Dox hepatotoxicity and associated apoptotic and necrotic cell death. The effects of SMN on Dox were broad, including the ability to modulate changes in both Bcl-xL and p53 expression. In animals treated with SMN, tissue Bcl-xL expression exceeded control values after Dox treatment. Taken together, these results demonstrated that SMN (i) reduced, delayed onset, or prevented toxic effects of Dox which are typically associated with hydroxyl radical production, (ii) performed as an antioxidant limiting oxidative stress, (iii) protected the integrity of the genome, and (iv) antagonized apoptotic and necrotic cell death while increasing antiapoptotic Bcl-xL protein levels and minimizing the leakage of proapoptotic cytochrome c from liver mitochondria. These observations demonstrate the protective actions of SMN in liver, and raise the possibility that such protection may extend to other organs during Dox treatment including the heart.

Signal transduction pathways involved in drug-induced liver injury

Hepatocyte death following drug intake is the critical event in the clinical manifestation of drug-induced liver injury (DILI). Traditionally, hepatocyte death caused by drugs had been attributed to overwhelming oxidative stress and mitochondria dysfunction caused by reactive metabolites formed during drug metabolism. However, recent studies have also shown that signal transduction pathways activated/inhibited during oxidative stress play a key role in DILI. In acetaminophen (APAP)-induced liver injury, hepatocyte death requires the sustained activation of c-Jun kinase (JNK), a kinase important in mediating apoptotic and necrotic death. Inhibition of JNK using chemical inhibitors or knocking down JNK can prevent hepatocyte death even in the presence of extensive glutathione (GSH) depletion, covalent binding, and oxidative stress. Once activated, JNK translocates to mitochondria, to induce mitochondria permeability transition and trigger hepatocyte death. Mitochondria are central targets where prodeath kinases such as JNK, prosurvival death proteins such as bcl-xl, and oxidative damage converge to determine hepatocyte survival. The importance of mitochondria in DILI is also observed in the Mn-SOD heterozygous (+/-) model, where mice with less mitochondrial Mn-SOD are sensitized to liver injury caused by certain drugs. An extensive body of research is accumulating suggesting a central role of mitochondria in DILI. Drugs can also cause redox changes that inhibit important prosurvival pathways such as NF-kappaB. The inhibition of NF-kappaB by subtoxic doses of APAP sensitizes hepatocyte to the cytotoxic actions of tumor necrosis factor (TNF). Many drugs will induce liver injury if simultaneously treated with LPS, which promotes inflammation and cytokine release. Drugs may be sensitizing hepatocytes to the cytotoxic effects of cytokines such as TNF, or vice versa. Overall many signaling pathways are important in regulating DILI, and represent potential therapeutic targets to reduce liver injury caused by drugs.

Pre-exposure to a novel nutritional mixture containing a series of phytochemicals prevents acetaminophen-induced programmed and unprogrammed cell deaths by enhancing BCL-XL expression and minimizing oxidative stress in the liver

From a disease-prevention perspective, recent progress in phytochemical and nutraceutical research clearly suggests (benefits outweigh the risk pattern). Although powerful antioxidant properties have been the most acclaimed mechanism of action for these entities, the individual antioxidants studied in clinical trials do not appear to have consistent preventative effects. The actions of the antioxidant nutrients alone do not explain the observed health benefits of diets rich in fruits and vegetables for chronic diseases. Therefore, we proposed that the additive and synergistic effects of phytochemicals in fruits and vegetables are responsible for these potent antioxidant and anticancer activities, and that the benefit of a diet rich in fruits and vegetables is attributed to the complex mixture of phytochemicals present in plants [1]. Surprisingly, however, no studies have attempted to evaluate the combined antitoxic potential of a phytochemical-nutraceutical mixture (PNM) in in vivo models. Therefore, this study, for the first time, was designed to investigate whether pre-exposure to a unique PNM has the ability to impede mechanistic events involved in acetaminophen (APAP)-induced hepatotoxicity. Besides several vitamins and minerals in balanced proportions (approximately US RDA), the PNM used in this investigation contained several well-known phytochemicals such as citrus flavonoids, red wine polyphenols, Garcinia, Gymnema, Ginkgo, Ephedra sinica, Camellia sinensis, Silybum, Guarana, Eluthero, Allium sativum and Ocimum basilicum extracts. To evaluate PNM's antitoxic potential, groups of animals ICR mice, 3 months old) received either a control diet or PNM containing diets (1X and 10X) for 4 weeks. On day-28, animals were divided into two subgroups. Half the animals were administered normal saline and the other half received 400mg/kg ip injections of APAP. All the animals were sacrificed 24h after APAP exposure. Serum and tissue (liver and kidneys) samples were analyzed. APAP alone caused massive liver injury (nearly 495-fold increase in ALT) and oxidative stress (Lipid peroxidation: 268% increase in MDA) coupled with genomic DNA fragmentation (288% increase). Exposure to 1X-PNM for 28 days significantly reduced animal mortality and all the APAP-induced biochemical events (In 1X-PNM + AP: ALT leakage decreased to 54 fold; MDA accumulation decreased to 125%, and DNA fragmentation decreased to 122%), whereas 10X-PNM + APAP slightly escalated both oxidative stress and genomic DNA fragmentation preceding liver injury. Liver homogenates subjected to western blot analysis disclosed the ability of 1X-PNM to counteract APAP-induced decrease in Bcl-xL expression. Histopathological evaluation of stained liver tissue sections indicated anti-apoptogenic and anti-necrogenic reponses coupled with near complete prevention of glycogen depletion by 1X-PNM. Collectively, our investigation suggests that a mixture containing an assortment of phytochemicals/nutraceuticals may serve as a much more powerful blend in preventing drug or chemical-induced organ injuries than a single phytochemical or nutraceutical entity. In addition, ephedra and caffeine containing PNM-exposure in a controlled manner may potentially shield vital target organs from toxicities caused by intentional, unintentional or accidental exposures to structurally and functionally diverse drug and chemical entities.

Differentiating the causes of metabolic acidosis in the poisoned patient

Numerous drugs and toxins may induce the development of a metabolic acidosis. The treating physician should be cognizant of the many compounds that can produce metabolic acidosis following an overdose or an accidental exposure, or with therapeutic use. Knowledge and comprehension of the substances associated with metabolic acidosis will facilitate the diagnosis and treatment of poisoned patients.

Metabolic acidosis: differentiating the causes in the poisoned patient

Metabolic acidosis may arise from several drugs and toxins through a variety of mechanisms. Differentiating the causes of metabolic acidosis in the poisoned patient is an indispensable skill in clinical practice. Comprehension of toxin-induced metabolic acidosis, combined with a thorough history, physical examination, appropriate use of laboratory tests, and a stepwise approach, should aid the clinician in determining the cause of metabolic acidosis in the poisoned patient. When confronted with such a patient, it is imperative that one administer appropriate antidotal therapy, when necessary, and provide the patient with exceptional supportive care.

Subliminal Fas stimulation increases the hepatotoxicity of acetaminophen and bromobenzene in mice

The hepatotoxicity of several drugs is increased by mild viral infections. During such infections, death receptor ligands are expressed at low levels, and most parenchymal cells survive. We tested the hypothesis that subliminal death receptor stimulation may aggravate the hepatotoxicity of drugs, which are transformed by cytochrome P-450 cytochrome P-450 into glutathione-depleting reactive metabolites. Twenty-four-hour-fasted mice were pretreated with a subtoxic dose of the agonistic Jo2 anti-Fas antibody (1 microg per mouse) 3 hours before acetaminophen (500 mg/kg) or 1 hour before bromobenzene (400 mg/kg) administration. Administration of Jo2 alone increased hepatic inducible nitric oxide synthase nitric oxide synthase but did not modify serum alanine aminotransferase (ALT), hepatic adenosine triphosphate (ATP), glutathione (GSH), cytochrome P-450, cytosolic cytochrome c, caspase-3 activity or hepatic morphology. However, pretreating mice with Jo2 further decreased both hepatic GSH and ATP by 40% 4 hours after acetaminophen administration, and further increased serum ALT and the area of centrilobular necrosis at 24 hours. In mice pretreated with the Jo2 antibody before bromobenzene administration, hepatic GSH 4 hours after bromobenzene administration was 51% lower than in mice treated with bromobenzene alone, and serum ALT activity at 24 hours was 47-fold higher. In conclusion, administration of a subtoxic dose of an agonistic anti-Fas antibody before acetaminophen or bromobenzene increases metabolite-mediated GSH depletion and hepatotoxicity. Subliminal death receptor stimulation may be one mechanism whereby mild viral infections can increase drug-induced toxicity.

Blood lactate as an early predictor of outcome in paracetamol-induced acute liver failure: a cohort study

BACKGROUND

Although the King's College Hospital (KCH) selection criteria for emergency liver transplantation in paracetamol-induced acute liver failure are widely used, strategies to improve sensitivity and facilitate earlier transplantation are required. We investigated the use of arterial blood lactate measurement for the identification of transplantation candidates.

METHODS

In a single-centre study, we measured arterial blood lactate early (median 4 h) and after fluid resuscitation (median 12 h) in patients admitted to a tertiary-referral intensive-care unit. Threshold values that best identified individuals likely to die without transplantation were derived in a retrospective initial sample of 103 patients with paracetamol-induced acute liver failure and applied to a prospective validation sample of 107 patients. Predictive value and speed of identification were compared with those of KCH criteria.

FINDINGS

In the initial sample, median lactate was significantly higher in non-surviving patients than in survivors both in the early samples (8.5 [range 1.7--21.0] vs 1.4 [0.53--7.9] mmol/L, p<0.0001) and after fluid resuscitation (5.5 [1.3--18.6] vs 1.3 [0.26--3.2], p<0.0001). Applied to the validation sample, a threshold value of 3.5 mmol/L early after admission had sensitivity 67%, specificity 95%, positive likelihood ratio 13, and negative likelihood ratio 0.35; the corresponding values for a threshold of 3.0 mmol/L after fluid resuscitation were 76%, 97%, 30, and 0.24. Combined early and postresuscitation lactate concentrations had similar predictive ability to KCH criteria but identified non-surviving patients earlier (4 [3--13] vs 10 [3.5--19.5] h, p=0.01). Addition of postresuscitation lactate concentration to KCH criteria increased sensitivity from 76% to 91% and lowered negative likelihood ratio from 0.25 to 0.10.

INTERPRETATION

Arterial blood lactate measurement rapidly and accurately identifies patients who will die from paracetamol-induced acute liver failure. Its use could improve the speed and accuracy of selection of appropriate candidates for transplantation.

Acetaminophen hepatotoxicity and mechanisms of its protection by N-acetylcysteine: a study of Hep3B cells

Acetaminophen (AAP) hepatotoxicity, resulting in centrilobular necrosis, is frequently encountered following suicidal attempts, especially by adolescents, but also after its excessive use in infants. The subcellular and molecular sequences leading to hepatocellular cell death are not yet clear. We therefore investigated AAP hepatotoxicity by using cultured hepatoma-derived cells (Hep3B) exposed to AAP and N-acetylcysteine (NAC), used as a protective agent. Specifically, we studied the role of apoptosis and oxidative damage as putative mechanisms of AAP-associated cytotoxicity. Hep3B cells were exposed to AAP (5-25 mM) and NAC (5 mM) for different time periods. Cell viability was assessed by the Alamar Blue Reduction Test and LDH. Oxidative damage was evaluated by measuring reactive oxygen species (ROS) and glutathione. AAP-induced apoptosis was investigated by flow cytometry and transmission electron microscopy. We found that: 1. In Hep3B cells, AAP causes a time- and concentration-dependent cytotoxic effect, leading to oxidative stress, mitochondrial dysfunction, alterations of membrane permeability and apoptosis; 2. In the course of AAP cytotoxicity, the generation of ROS appears as an early event which precedes decrease of viability, LDH leakage, glutathione depletion and apoptosis; 3. NAC protects Hep3B cells from AAP-induced oxidative injury, but does not prevent apoptosis.

Ca(2+)-calmodulin antagonist chlorpromazine and poly(ADP-ribose) polymerase modulators 4-aminobenzamide and nicotinamide influence hepatic expression of BCL-XL and P53 and protect against acetaminophen-induced programmed and unprogrammed cell death in mice

Acetaminophen (AAP), the analgesic hepatotoxicant, is a powerful inducer of oxidative stress, DNA fragmentation, and apoptosis. The anti-apoptotic oncogene bcl-XL, and the pro-apoptotic oncogene p53 are two key regulators of cell cycle progression and/or apoptosis subsequent to DNA damage in vitro and in vivo. This study investigated the effect of AAP on the expression of these oncogenes and whether agents that modulate DNA fragmentation (chlorpromazine, CPZ) and DNA repair through poly(ADP-Ribose) polymerase (PARP) activity (4-AB: 4-aminobenzamide) can protect against AAP-induced hepatotoxicity by inhibiting oxidative stress, DNA fragmentation, and/or by altering the expression of bcl-XL and p53. In addition, the protective effect of supplemental nicotinamide (NICO), known to be depleted in cells with high PARP activity during DNA repair, is similarly evaluated. Male ICR mice (3 months old) were administered vehicle alone; nontoxic doses of 4-AB (400 mg/kg, ip), NICO (250 mg/kg, ip) or CPZ (25 mg/kg, ip), hepatotoxic dose of AAP alone (500 mg/kg, ip), or AAP plus one of the protective agents 1 h later. All animals were sacrificed 24 h following AAP administration. Serum alanine aminotransferase activity (ALT), hepatic histopathology and lipid peroxidation, DNA damage, and expression of bcl-XL and p53 (western blot analysis) were compared in various groups. All of the three agents significantly prevented AAP-induced liver injury, lipid peroxidation, DNA damage, and associated apoptotic and necrotic cell deaths, 4-AB being the most effective and NICO the least. Compared to control, there was a considerable decrease in bcl-XL expression, and an increase in p53 expression in AAP-exposed livers. The effect of AAP on bcl-XL was antagonized and that on p53 was synergized by the PARP-modulator 4-AB as well as NICO, whereas the endonuclease inhibitor CPZ was without effect on either bcl-XL or p53 expression. These results suggest that the hepatotoxic effect of AAP involves multiple mechanisms including oxidative stress, upregulation of endonuclease (or caspase-activated DNAse) and alteration of pro- and anti-apoptotic oncogenes. The observed antagonism of AAP-induced hepatocellular apoptosis and/or necrosis by modulators of multiple processes including DNA repair suggests the likelihood that a more effective therapy against AAP intoxication should involve a combination of antidotes.

EC-SOD gene therapy reduces paracetamol-induced liver damage in mice

BACKGROUND

Paracetamol overdose causes acute liver damage which leads to severe centrilobular hepatic necrosis. The hepatotoxic effect is caused by reactive metabolites and oxidative stress. Since extracellular superoxide dismutase (EC-SOD) protects tissues against the harmful effects of superoxide anion, the hypothesis that systemic adenovirus-mediated EC-SOD gene transfer could reduce liver damage was tested.

METHODS

Mice were given paracetamol (600 mg/kg) enterally 2 days after adenovirus-mediated gene transfer of EC-SOD (2 x 10(9) pfu). Five days after gene transfer, plasma and tissue samples were collected for clinical chemistry analyses and tissue pathology evaluation.

RESULTS

EC-SOD was expressed in a dose-dependent manner with the highest enzyme activity occurring 3 days after the gene transfer. Clinical chemistry and tissue pathology analyses showed that adenoviral EC-SOD gene transfer significantly attenuated release of liver enzymes and inhibited necrosis and apoptosis caused by paracetamol overdose.

CONCLUSION

The results indicate the involvement of superoxide anion in paracetamol-mediated liver damage and suggest a possible protective role for EC-SOD gene transfer in paracetamol-induced liver damage.

Advances in molecular toxicology-towards understanding idiosyncratic drug toxicity

Idiosyncratic drug toxicity is a major complication of drug therapy and drug development. Such adverse drug reactions (ADRs) include anaphylaxis, blood dyscrasias, hepatotoxicity and severe cutaneous reactions. They are usually serious and can be fatal. At present, prediction of idiosyncratic ADRs at the preclinical stage of drug development is not possible because there are no suitable animal models and we do not understand the basic mechanisms involved in the toxicity when it does occur in man. Many idiosyncratic reactions appear to have an immunological aetiology. For example, there is increasing evidence for the role of T lymphocytes in severe skin reactions. Nevertheless, the sequence of events by which a simple chemical can elicit severe tissue damage remains poorly understood and alternative novel mechanisms of toxicity must also be explored. The purpose of this article will be to review the currently accepted mechanisms of idiosyncratic drug toxicity at the chemical and the molecular levels. In particular, we will consider how recent advances in cellular immunology and molecular biology can improve our understanding of both the chemical and clinical aspects of drug hypersensitivity. Recent advances in the role of both inter- and intra-cellular signalling in the regulation of the immune response to drugs and their metabolites will be discussed. The long-term aim of such research is to provide test systems for the evaluation of drug safety and patient susceptibility to idiosyncratic drug toxicity.

The hepatoprotective effects of Solanum alatum Moench. on acetaminophen-induced hepatotoxicity in mice

Solanum alatum Moench. has been shown to have a protective effect against carbon tetrachloride (CCl4)-induced liver injury. Solanum alatum treatment (100 mg/kg, p.o.) decreased the elevation of serum alanine aminotransferase (ALT; GPT) and aspartate aminotransferase (AST; GOT) induced by acetaminophen (paracetamol) (600 mg/kg, i.p.) administration. It also decreased the extent of visible necrosis in liver tissue. In addition, Solanum alatum treatment restored hepatic glutathione (GSH) depletion induced by acetaminophen (600 mg/kg, i.p.) administration. Microsomal enzyme levels such as P-450, reductase, and aniline hydroxylation enzyme were also restored to normal levels after Solanum alatum administration. The hepatoprotective mechanism may function through direct binding with acetaminophen toxic metabolites, decreasing the attraction of acetaminophen metabolites for other cellular GSH or thiol protein. Additionally, Solanum alatum treatment increased the concentration of hepatic GSH and maintained a high level activity of GSTase, which led to acceleration of the excretion of toxic acetaminophen metabolites.

Metabolic acidosis and coma following a severe acetaminophen overdose

OBJECTIVE

To report a case of metabolic acidosis and coma in a severe acetaminophen overdose.

CASE SUMMARY

A 29-year-old white woman was admitted to the emergency department with a diminished level of consciousness and metabolic acidosis. The toxicology screen revealed a serum acetaminophen concentration of 1072 microg/mL, and she was consequently treated with intravenous acetylcysteine. Despite the elevated concentration, the patient did not manifest signs of hepatotoxicity.

DISCUSSION

Metabolic acidosis and coma are rare manifestations in acetaminophen overdoses. In published case reports, severe acetaminophen ingestion independently causes metabolic acidosis and coma in the absence of hepatotoxicity. The mechanism by which metabolic acidosis occurs is not clearly defined. Studies conducted on animals demonstrated that in severe overdoses, acetaminophen may cause lactic acidosis by inhibiting mitochondrial respiration. The mechanism by which acetaminophen can cause coma is still unknown.

CONCLUSIONS

Severe acetaminophen overdoses can independently cause metabolic acidosis and coma in the absence of hepatotoxicity.

A novel proanthocyanidin IH636 grape seed extract increases in vivo Bcl-XL expression and prevents acetaminophen-induced programmed and unprogrammed cell death in mouse liver

Several molecular events in the apoptotic or necrotic death of hepatocytes induced by acetaminophen (AAP) now appear to be well defined. Recent studies also indicate that select expression of bcl-Xl is possibly modified during AAP-induced liver injury. The purpose of this study was several-fold: (i) to examine the hepatoprotective ability of short-term (3-day) and long-term (7-day) exposures of a grape seed proanthocyanidin extract (GSPE) on AAP-induced liver injury and animal lethality; (ii) to monitor effects of GSPE on one of the prime targets of AAP, i.e., hepatocellular genomic DNA and associated apoptotic and necrotic death; and (iii) to unravel changes in the pattern of expression of an antiapoptotic gene, bcl-Xl in the liver. In order to investigate these events, male ICR mice (30-40 g) were administered nontoxic doses of GSPE (3 or 7 days, 100 mg/kg, po), followed by hepatotoxic doses of AAP (400 and 500 mg/kg, ip), and sacrificed 24 h later. Serum was analyzed for alanine aminotransferase activity (ALT) and the liver for histopathological diagnosis of apoptosis/necrosis. The ability of AAP to promote apoptotic DNA fragmentation and its counteraction by GSPE in the liver was also evaluated quantitatively (by a sedimentation assay) and qualitatively (by agarose gel electrophoresis). Portions of livers were also subjected to Western blot analysis (27,000g fraction of liver homogenates) to examine the pattern of expression of cell death inhibitory gene bcl-Xl. Results indicate that 7-day GSPE preexposure induced dramatic protection and markedly decreased liver injury and animal lethality culminated by AAP, when compared to a short-term 3-day exposure. Abrogation of toxicity was also mirrored in DNA fragmentation. Histopathological evaluation of liver sections showed remarkable counteraction of AAP-toxicity by this novel GSPE and substantial inhibition of both apoptotic and necrotic liver cell death. Agarose gel electrophoresis revealed that 7-day GSPE preexposure prior to AAP administration completely blocked Ca(2+)/Mg(2+)-Ca(2+)/Mg(2+)-dependent-endonuclease-mediated ladder-like fragmentation of genomic DNA and significantly altered the bcl-Xl expression. The most dramatic changes observed in this study were: (i) substantial increase in the expression of bcl-Xl in the liver by 7-day GSPE exposure alone; (ii) significant modification bcl-Xl expression by AAP alone; and (iii) dramatic inhibition of AAP-induced modification of bcl-Xl (phosphorylation?) expression by GSPE. In summary, these observations demonstrate that GSPE preexposure may significantly attenuate AAP-induced hepatic DNA damage, apoptotic and necrotic cell death of liver cells, and, most remarkably, antagonize the influence of AAP-induced changes in bcl-Xl expression in vivo.

Inhibition of Fas receptor (CD95)-induced hepatic caspase activation and apoptosis by acetaminophen in mice

The mechanism of liver cell injury induced by an overdose of the analgesic acetaminophen (AAP) remains controversial. Recently, it was hypothesized that a significant number of hepatocytes die by apoptosis. Since caspases have been implicated as critical signal and effector proteases in apoptosis, we investigated their potential role in the pathophysiology of AAP-induced liver injury. Male C3Heb/FeJ mice were fasted overnight and then treated with 500 mg/kg AAP. Liver injury became apparent at 4 h and was more severe at 6 h (plasma ALT activities: 4110 +/- 320 U/liter; centrilobular necrosis). DNA fragmentation increased parallel to the increase of plasma ALT values. At 6 h there was a 420% increase of DNA fragmentation and a 74-fold increase of terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL)-positive cells located predominantly around central veins. However, the activity of the proapoptotic caspase-3 was not increased at any time after AAP. In contrast, injection of the anti-Fas antibody Jo-2 (positive control) caused a 28-fold increase of caspase-3 activity and severe DNA fragmentation before significant ALT release. Treatment with the caspase inhibitor ZVAD-CHF2 had no effect on AAP toxicity but completely prevented Jo-mediated apoptosis. In contrast, Jo-induced caspase activation and apoptosis could be inhibited by AAP treatment in a time- and dose-dependent manner. We conclude that AAP-induced DNA fragmentation does not involve caspases, suggesting a direct activation of endonucleases through elevated Ca2+ levels. In addition, electrophilic metabolites of AAP may inactivate caspases or their activation pathway. This indicates that AAP metabolism has the potential to inhibit signal transduction mechanisms of receptor-mediated apoptosis.

N-acetylcysteine enhances endothelium-dependent vasorelaxation in the isolated rat mesenteric artery

STUDY HYPOTHESIS

Previous studies have suggested that N-acetylcysteine (NAC) may confer additional protection in acetaminophen (APAP) overdose by improving hepatic microcirculation. We hypothesize that NAC enhances release of nitric oxide (NO) from the vasculature.

METHODS

Sprague-Dawley rat superior mesenteric artery rings were suspended in oxygenated Krebs-Henseleit tissue baths and contracted with U-46619 (a thromboxane A2-mimetic). In part 1, the effect of NAC on endothelial cell (EC) release of NO was assessed by measurement of vasorelaxation induced by acetylcholine (ACh, an EC-dependent vasorelaxor) in the presence and absence of NAC. In part 2, the effect of glutathione (a major component of NAC hepatoprotection) was examined by measuring ACh-induced vasorelaxation in rings from rats treated with L-buthionine sulfoxamine (BSO, a glutathione synthesis inhibitor). Data were analyzed by repeated-measures ANOVA.

RESULTS

Addition of 15 to 30 mmol/L NAC after ring contraction had no direct vasodilatory effect. By contrast, pretreatment of rings with NAC (15 mmol/L) enhanced vasorelaxation induced by ACh (95.0% +/- 7.9% versus 62.3% +/- 7.6% for control; ACh dose, 1 mumol/L; P < .001) or by A23187, a receptor-independent, NO-mediated vasodilator (91.6% +/- 9.6% versus 68.3% +/- 12.1% for control; A23187 dose, 1 mumol/L; P < .001). In rings from BSO-treated rats, NAC also enhanced vasorelaxation (76.5% +/- 7.1%; P < .001 versus control), but to a lesser degree than in nontreated rats.

CONCLUSION

NAC enhances endothelium-dependent vasodilation in an isolated rat mesenteric artery ring preparation. In addition to its antioxidant effects, NAC may decrease APAP hepatotoxicity by stimulating NO production and improving microvascular circulation.

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.

Effects of combined use of diallyl disulfide and Nacetyl-cysteine on acetaminophen hepatotoxicity in beta-naphthoflavone pretreated mice

AIM:To assess the protective effect of diallyl disulfide (DADS) and its combined use with N-acetyl-cysteine (NAC) on acetaminophen (APAP) hepatotoxicity in C57BL/6N (B6) mice pretreated with beta-naphthoflavone (BNF).METHODS:B6 mice were divided into six groups and all compounds used were injected intraperitoneally. Except for control and APAP group (receiving APAP only), the other groups received an injection of APAP (350mg/kg) 48 hours after BNF (200mg/kg) and either of DADS (200mg/kg), or NAC (500mg/kg) or both DADS and NAC.DADS was given 2 hours before APAP and NAC was injected with APAP.The mean survival time was recorded and livers were examined histologically.Hepatic glutathione (GSH) levels and plasma ALT were also determined at different time points.To evaluate the effect of DADS or NAC on hepatic P450 induction by BNF,liver microsomes were prepared and 7-ethoxyresorufin O-dealkylase (ERD) activity was determined using spectrofluorometrical methods. In vitro effect of DADS or NAC on ERD activity was assayed by directly incubating microsomal suspension with DADS or NAC of different concentrations.RESULTS:APAP was not toxic to mice without BNF pretreatment, but caused severe liver necrosis and death of all BNF-treated mice in 4 hours. A sharp depletion of GSH (approximately 62% of its initial content at 2 hours and 67% at 4 hours) and a linear elevation of ALT levels (536.8 plus minus 29.5 Sigma units at 2 hours and 1302.5 plus minus74.9 at 4 hours) were observed.DADS and NAC given individually produced mild protection,resulting in prolonged survival,a slower decline of GSH level and a less steeper elevation of ALT level.All mice died eventually. Co-administration of DADS and NAC completely protected mice.GSH level in this group lowered by about 35% and 30% at 2 and 4 hours, and ALT was 126 plus minus 18 and 157.5 plus minus 36.6 Sigma units at 2 and 4 hours. ERD activity in BNF-treated mice was about 5 times that of the constitutive level determined in normal mice. Neither DADS nor NAC inhibited P450 1A1/1A2 induction as determined by their effect on the induction of ERD activity.In vitro assay indicates that DADS,but not NAC,was a potent inhibitor of ERD activity(IC(50) = 4.6&mgr;m).CONCLUSION:A combined use of both DADS and NAC produced full protection in BNF treated mice against APAP hepatotoxicity.The mechanism is that DADS inhibits P450 1A1/1A2 activity, but not induction, which substantially reduces production of NAPQI, while NAC enhances liver detoxifying capability via serving as a precursor of GSH and stimulating GSH synthesis.

Acetaminophen cytotoxicity in mouse eye: mitochondria in anterior tissues are the primary target

Acetaminophen (APAP) injected into C57BL/6 mice (cytochrome P450 inducer-responsive strain) that had been pretreated with b-naphthoflavone (BNF) produced ocular tissue damage, including cataract. Our previous histocytochemical studies showed that tissue damage spread in association with the flow of the aqueous humor and appeared first in the ciliary epithelium, followed by the iris and corneal endothelium and, finally, the lens. The neural retina, retinal pigmented epithelium and choroid remained unaffected. A close examination of the affected tissues indicated that mitochondria are the primary target of APAP cytotoxicity. In order to investigate whether the respiratory capacity of mitochondria is more sensitive to APAP cytotoxicity than mitochondrial morphology, we determined in this work the oxygen uptake by eye tissues dissected from BNF-pretreated and APAP-injected C57BL/6 mice. Oxygen uptake by the ciliary body/iris decreased about 60% at 90 min and 85% at 120 min after APAP administration. The oxygen uptake was inhibited about 50% by 10 microM rotenone. Since the earliest sign of mitochondrial damage was noted at 120 min, the result indicates that mitochondrial energy dysfunction precedes morphological alterations. It was also observed that oxygen uptake by the retina remained unaffected at least for 120 min after APAP administration; therefore, it is evident that the retina and, possibly, other posterior tissues as well are resistant to APAP cytotoxicity, not only in their morphology but, also, in their capacity of mitochondrial energy metabolism.

Furan-mediated uncoupling of hepatic oxidative phosphorylation in Fischer-344 rats: an early event in cell death

Furan is a potent rodent hepatotoxicant and carcinogen. The present study was done to examine the effects of furan on hepatic energy metabolism both in vivo and in vitro in male F-344 rats. Furan produced concentration- and incubation time-dependent irreversible reductions in ATP in freshly isolated F-344 rat hepatocytes. Furan-mediated depletion of ATP occurred prior to cell death and was prevented by including 1-phenylimidazole, a cytochrome P450 inhibitor, in the suspensions. Male F-344 rats were treated with furan (0-30 mg/kg, po) and killed 24 hr later to prepare hepatic mitochondria. Furan produced dose-dependent increases in state 4 respiration and ATPase activity. Both of these changes were prevented by 1-phenylimidazole cotreatment. In a separate series of experiments, mitochondria were prepared from isolated rat hepatocytes following incubation with furan (2-100 microM) for 1-4 hr. Furan produced incubation time- and concentration-dependent increases in state 4 respiration and ATPase activity. Furan-mediated mitochondrial changes were prevented by adding 1-phenylimidazole to the hepatocyte suspensions. These results indicate that the ene-dialdehyde metabolite of furan uncouples hepatic oxidative phosphorylation in vivo and in vitro. In vitro studies using an isolated hepatocyte suspension/culture system demonstrated that the concentration response for furan-mediated mitochondrial changes in suspension corresponded with the concentration responses for cell death after 24 hr. Including 1-phenylimidazole or oligomycin plus fructose in hepatocyte suspensions prevented furan-induced cell death after 24 hr in culture. The results of this study indicate that furan-induced uncoupling of oxidative phosphorylation is an early, critical event in cytolethality both in vivo and in vitro.

Acetaminophen-arylated proteins are detected in hepatic subcellular fractions and numerous extra-hepatic tissues in CD-1 and C57B1/6J mice

To identify acetaminophen (APAP)-bound proteins in addition to the major 44 and 58 kDa APAP-binding proteins (Bartolone et al., 1992, Toxicol. Appl. Pharmacol. 113. 19-9; Pumford et al., 1992, Biochem. Biophys. Res. Commun. 182, 1348-1355; Bulera et al., 1995, Toxicol, Appl. Pharmacol. 134, 313-320), we investigated subcellular localization of liver proteins and tissue distribution of proteins arylated by a hepatotoxic dose of APAP in CD-1 and C57B1/6J mice. Western blot analysis with affinity-purified, anti-APAP antibodies allowed the detection of covalently bound proteins in liver mitochondria, nuclei, membrane, cytosol, and microsomes. Enzyme market assays revealed that subcellular fractions were 90-98% pure. The lack of contamination from other isolated subcellular fractions indicates that covalently bound proteins were specific to the particular subcellular fraction. APAP-arylated proteins with molecular weights similar to those detected in the liver were found in cytosolic fractions from kidney, lung, pancreas, heart, skeletal muscle, and stomach. The presence of arylated proteins in extra-hepatic organs suggests that other organs may be susceptible to APAP toxicity and may contain critical protein targets that are important in APAP toxicity. In contrast, covalently bound proteins were not detected in cytosols isolated from spleen, small intestine, brain, and testis. The characterization of the APAP-arylated proteins identified in this study will aid in elucidating the mechanism of APAP-induced toxicity.

Comparison of protection by fructose against paracetamol injury with protection by glucose and fructose-1,6-diphosphate

We have compared the protective effect of fructose in normal Ringer solution during the onset and progression of cell injury induced by paracetamol in rat liver slices with the protective effect of glucose and fructose-1,6-diphosphate. Liver slices obtained from phenobarbitone-induced and non-induced rats were used in a model in vitro system. Slices were exposed to 10 mM paracetamol for 120 min and then incubated without paracetamol in the presence or absence of protective agents for a further 240 min. Cell injury was quantified by measuring leakage of lactate dehydrogenase (LDH) and potassium (K+). Adenosinetriphosphate (ATP) levels were measured using the luciferin-luciferase bioluminescence assay. Addition of higher concentrations of glucose (10-50 mM) to Ringer solution were not found to result in protection at the end of incubation in paracetamol-treated slices obtained from phenobarbitone-induced rats. Neither did sucrose nor mannitol protect. However, exclusion of glucose from Ringer solution resulted in cell injury in paracetamol-treated slices obtained from non-induced rats. Methionine, a known antidote for paracetamol poisoning, failed to protect in this instances but fructose did protect. This suggests that the presence of a glycolytic substrate plays a crucial role in cell protection. Further evidence for this is the finding that iodoacetate, an inhibitor of glycolysis, not only increase cell injury in paracetamol-treated slices but also reverses fructose protection. Fructose-1,6-diphosphate was found to protect against the onset and progression of cell injury in paracetamol-treated slices obtained from phenobarbitone induced rats. This protective agent is found to maintain high ATP levels and cell viability in paracetamol-treated slices at a time when paracetamol-treated slices show a profound loss of ATP levels and a significant increase in cell injury as measured by leakage of LDH and K+.

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.

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)

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.