The toxicity of acetaminophen and N-acetyl-p-benzoquinone imine in isolated hepatocytes is associated with thiol depletion and increased cytosolic Ca2+

Acetaminophen bioactivation by human cytochrome P450 enzymes and animal microsomes

Acetaminophen is a widely used analgesic antipyretic agent. When used at low doses, it is a safe drug, but at higher doses it can cause acute hepatic necrosis in humans and experimental animals. The key mechanism in the hepatotoxicity is cytochrome P450 (CYP)-catalysed formation of the reactive metabolite, N-acetyl-p-benzoquinone imine (NAPQI) that is capable of binding to cellular macromolecules and in that way an LC/MS liquid chromatography/mass spectrometry (LC/MS) method was developed to measure NAPQI formation by trapping it to reduced glutathione. This method was used to determine the bioactivation of acetaminophen at two concentrations: 50 microM therapeutic and 1 mM toxic by using nine human recombinant CYP enzymes: CYP1A1, CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4; and with different microsomes from experimental animals. At the toxic concentration the formation of NAPQI-glutathione was highest with CYP3A4 followed by CYP2E1, CYP1A2, and CYP2D6. At the therapeutic concentration, CYP3A4 had also the highest bioactivation capacity. In a comparison of the enzyme kinetics, CYP3A4 was the most efficient CYP with the lowest K(m) value 130 microM (95% confidence interval = 63-210 microM). Dexamethasone-induced rat liver microsomes had the most effective bioactivation capacity at therapeutic and toxic acetaminophen concentrations. This study suggests that CYP3A4 is the major CYP enzyme form catalysing acetaminophen oxidation to NAPQI in human liver.

Understanding the role of reactive metabolites in drug-induced hepatotoxicity: state of the science

Drug-induced liver injury (DILI) represents a major impediment to the development of new drugs and is a leading cause of drug withdrawal. The occurrence of hepatotoxicity has been closely associated with the formation of chemically reactive metabolites. Huge investment has focused on the screening of chemically reactive metabolites to offer a pragmatic approach to produce safer drugs and also reduce drug attrition and prevent market place withdrawal. However, questions surrounding the importance of chemically reactive metabolites still remain. Increasing evidence now exists for the multi-factorial nature of DILI, in particular the role played by the host immune system or disease state in the pathogenesis of DILI. This review aims to evaluate the current measures for the prediction and diagnosis of DILI and to highlight investigations being made to understand the multidimensional nature. Some of the steps being made to generate improved physiological systems to identify more sensitive, reflective mechanism-based biomarkers to aid the earlier identification of DILI and develop safer medicines are also discussed.

Reduced acetaminophen-induced liver injury in mice by genetic disruption of IL-1 receptor antagonist

Acetaminophen (APAP) induced increases in intrahepatic expression of interleukin (IL)-1 alpha, IL-1 beta, and IL-1 receptor antagonist (IL-1ra), when administered intraperitoneally. These observations prompted us to define the pathophysiological roles of IL-1ra in APAP-induced liver injury. Compared with wild-type (WT) mouse-derived hepatocytes, IL-1ra-deficient (IL-1ra KO)-derived hepatocytes exhibited more resistance against APAP but not APAP-derived major toxic metabolite, N-acetyl-p-benzoquinone imine (NAPQI). Moreover, the amounts of a major APAP adduct (selenium-binding protein), an indicator of NAPQI generation from APAP, was significantly lower in IL-1ra KO mice than WT mice with depressed intrahepatic expression of CYP1A2, CYP2E1, and CYP3A11, the enzymes crucially involved in NAPQI generation from APAP. These observations would indicate that IL-1ra deficiency impaired APAP metabolism. IL-1 alpha and IL-1 beta were expressed to similar extents in livers of untreated IL-1ra KO and WT mice. By contrast, the intranuclear amount of p65 of NF-kappaB, which can suppress the gene expression of CYP1A2, CYP2E1, and CYP3A11, was higher in untreated IL-1ra KO than WT mice. Moreover, when mice were intraperitoneally administered APAP (200 mg/kg), IL-1ra KO mice exhibited attenuated APAP-induced liver injury as evidenced by reductions in serum alanine transferase levels and histopathological changes such as centrilobular necrosis, hemorrhages, and leukocyte infiltration. Finally, when given 12 h before APAP challenge, IL-1 alpha repressed the intrahepatic expression of CYP1A2, CYP2E1, and CYP3A11, eventually reducing APAP-induced liver injury, along with reduction in APAP adducts. Collectively, NF-kappaB was activated without any stimuli by the genetic disruption of IL-1ra, and suppressed cytochrome P450 enzyme expression, thereby reducing APAP-induced liver injury.

Protective effects of salidroside against acetaminophen-induced toxicity in mice

The protective effect of salidroside (SDS) isolated from Rhodiola sachalinensis A. BOR. (Crassulaceae), was investigated in acetaminophen (APAP)-induced hepatic toxicity mouse model in comparison to N-acetylcysteine (NAC). Drug-induced hepatotoxicity was induced by an intraperitoneal (i.p.) injection of 300 mg/kg (sub-lethal dose) of APAP. SDS was given orally to mice at a dose of 50 or 100 mg/kg 2 h before the APAP administration in parallel with NAC. Mice were sacrificed 12 h after the APAP injection to determine aspartate aminotransferase (AST), alanine aminotransferase (ALT), and tumor necrosis factor-alpha (TNF-alpha) levels in serum and glutathione (GSH) depletion, malondialdehyde (MDA) accumulation, and caspase-3 expression in liver tissues. SDS significantly protected APAP-induced hepatotoxicity for SDS improved mouse survival rates better than NAC against a lethal dose of APAP and significantly blocked not only APAP-induced increases of AST, ALT, and TNF-alpha but also APAP-induced GSH depletion and MDA accumulation. Histopathological and immunohistochemical analyses also demonstrated that SDS could reduce the appearance of necrosis regions as well as caspase-3 and hypoxia inducible factor-1alpha (HIF-1alpha) expression in liver tissue. Our results indicated that SDS protected liver tissue from the APAP-induced oxidative damage via preventing or alleviating intracellular GSH depletion and oxidation damage, which suggested that SDS would be a potential antidote against APAP-induced hepatotoxicity.

Acetaminophen safety and hepatotoxicity--where do we go from here?

Acetaminophen has been widely used for > 50 years in the treatment of pain and fever and provides for the safe and effective relief of these symptoms. In a small minority of patients, however, acetaminophen is responsible for life-threatening liver injury and accounts for up to 50% of all adult cases of acute liver failure in the US. Although approximately two-thirds of adult overdoses are associated with suicide attempts, many are inadvertent, often due to the use of multiple acetaminophen formulations over many days. Additionally, some individuals appear to experience acetaminophen toxicity at 'therapeutic' doses of < 4 g/day, for reasons unknown. In pediatric populations, the overwhelming majority of acetaminophen overdoses are due to unintentional overdoses, except for the predominance of suicidal ingestions in the teenage population. This article seeks to review the mechanism and metabolism of acetaminophen and the features of toxicity in adults, pediatric and special populations. Additionally, expert opinion is presented herein to aid in reducing the frequency and severity of liver injury from acetaminophen.

Hepatoprotective effects of Chai-Hu-Ching-Kan-Tang on acetaminophen-induced acute liver injury in rats

Chai-Hu-Ching-Kan-Tang (CHCKT) is one of the traditional Chinese medicine prescriptions commonly used to treat liver diseases. In this study, we evaluated the hepatoprotective effects of aqueous CHCKT extract at various concentrations (125, 250 and 500 mg/kg body weight) on acetaminophen (APAP)-induced acute liver injury in rats. Results showed that CHCKT treatments significantly decreased the level of serum glutamic oxaloacetic transaminase (sGOT) and glutamic pyruvic transaminase (sGPT) in APAP-treated groups. CHCKT also significantly decreased the level of lipid peroxides and increased the activity of antioxidant enzymes (i.e. SOD and GPx). Histopathological observation further confirmed the hepatoprotective activity of CHCKT as indicated by the amelioration in the central necrosis and fatty changes of the liver after APAP induction. Interestingly, the hepatoprotective activity of CHCKT at concentrations 125~500 mg/kg appeared to be as good as 12.5 mg/kg silymarin (a commercial hepatoprotective agent). Taken together, these results suggest that aqueous extract of CHCKT possesses potent hepatoprotective effects agianst APAP-induced liver injury in rats.

Simulation of the detoxification of paracetamol using on-line electrochemistry/liquid chromatography/mass spectrometry

On-line electrochemistry/liquid chromatography/mass spectrometry was used to simulate the detoxification mechanism of paracetamol in the body. In an electrochemical flow-through cell, paracetamol was oxidized at a porous glassy carbon working electrode at a potential of 600 mV vs. Pd/H2 with formation of a quinoneimine intermediate. The quinoneimine further reacted with glutathione and/or N-acetylcysteine to form isomeric adducts via the thiol function. The adducts were characterized on-line by liquid chromatography/mass spectrometry. These reactions are similar to those occurring between paracetamol and glutathione under catalysis by cytochrome P450 enzymes in the body.

Aggregates of ultrafine particles impair phagocytosis of microorganisms by human alveolar macrophages

We investigated whether exposure of alveolar macrophages to aggregates of ultrafine carbon particles affected subsequent phagocytosis of microorganisms. Human alveolar macrophages were obtained by bronchoalveolar lavage and exposed to aggregates of ultrafine carbon particles or diesel exhaust particles (DEP) for 20 h before measurements of phagocytosis. The particle loads were estimated to be comparable to those of air pollution exposure with established health effects in humans. Phagocytotic activity was measured as attachment and ingestion of four different test particles (amorphous silica particles, yeast cells from Candida albicans, and Cryptococcus neoformans opsonized with specific IgG or fresh serum) that bind to scavenger, mannose, Fc, and complement receptors, respectively. Carbon preloading significantly impaired the attachment and ingestion process (P<0.01) for all particles, except for yeast cells from C. neoformans opsonized with specific IgG. On the average, the accumulated attachment decreased by 30% and the ingested fraction decreased by 10%. Loading of alveolar macrophages with either aggregates of ultrafine DEP or carbon particles impaired the phagocytosis of silica test particles in a similar way. Exposure of human alveolar macrophages to aggregates of carbon or DEP, in concentrations relevant to human environmental exposures, caused significant impairment of phagocytosis of silica particles and microorganisms. The inhibitory effect on particle phagocytosis mediated by four different receptors suggests that air pollution particles cause a general inhibition of macrophage phagocytosis. Such an effect may contribute to increased susceptibility to infections and, for example, result in more exacerbations of asthma and chronic obstructive pulmonary disease.

Chemistry and hepatoprotective activity of an active fraction from Barleria prionitis Linn. in experimental animals

Iridoid enriched fraction IF from the ethanol-water extract of aerial parts (leaves and stems) of Barleria prionitis Linn. was evaluated for hepatoprotective activity in various acute and chronic animal test models of hepatotoxicity. It afforded significant hepatoprotection against carbon tetrachloride, galactosamine and paracetamol induced hepatotoxicity. Silymarin, was used as reference hepatoprotective. In the safety evaluation study the oral LD50 was found to be more than 3000 mg/kg, with no signs of abnormalities or any mortality observed for 15 days period under observation after single dose of drug administration whereas intraperitoneal LD50 was found to be 2530 mg/kg+/-87 mg/kg. SE (n=10) in mice. The studies revealed significant and concentration dependent hepatoprotective potential of 'IF' as it reversed the majority of the altered hepatic parameters in experimental liver damage in rodents.

HIP/PAP accelerates liver regeneration and protects against acetaminophen injury in mice

Human hepatocarcinoma-intestine-pancreas/pancreatic-associated protein HIP/PAP is a secreted C-type lectin belonging to group VII, according to Drickamer's classification. HIP/PAP is overexpressed in liver carcinoma; however, its functional role remains unclear. In this study, we demonstrate that HIP/PAP is a paracrine hepatic growth factor promoting both proliferation and viability of liver cells in vivo. First, a low number of implanted hepatocytes deriving from HIP/PAP-transgenic mice (<1:1,000) was sufficient to stimulate overall recipient severe combined immunodeficiency liver regeneration after partial hepatectomy. After a single injection of HIP/PAP protein, the percentages of bromodeoxyuridine-positive nuclei and mitosis were statistically higher than after saline injection, indicating that HIP/PAP acts as a paracrine mitogenic growth factor for the liver. Comparison of the early events posthepatectomy in control and transgenic mice indicated that HIP/PAP accelerates the accumulation/degradation of nuclear phospho-signal transducer activator transcription factor 3 and tumor necrosis factor alpha level, thus reflecting that HIP/PAP accelerates liver regeneration. Second, we showed that 80% of the HIP/PAP-transgenic mice versus 25% of the control mice were protected against lethal acetaminophen-induced fulminate hepatitis. A single injection of recombinant HIP/PAP induced a similar cytoprotective effect, demonstrating the antiapoptotic effect of HIP/PAP. Comparison of Cu/Zn superoxide dismutase activity and glutathione reductase-like effects in control and transgenic liver mice indicated that HIP/PAP exerts an antioxidant activity and prevents reactive oxygen species-induced mitochondrial damage by acetaminophen overdose. In conclusion, the present data offer new insights into the biological functions of C-type lectins. In addition, HIP/PAP is a promising candidate for the prevention and treatment of liver failure.

Studies on the antihypertensive, antispasmodic, bronchodilator and hepatoprotective activities of the Carum copticum seed extract

This study describes the antihypertensive, antispasmodic, bronchodilator and hepatoprotective activities of the aqueous-methanolic extract of Carum copticum Benth. seeds (CSE) to rationalize some of its traditional uses. CSE (3-100 mg/kg) caused a dose-dependent fall in arterial blood pressure in anaesthetized rats. In isolated rabbit aorta and jejunum preparations, CSE (0.1-3.0 mg/ml) caused an inhibitory effect on the K+-induced contractions. The calcium channel blocking (CCB) effect was confirmed when CSE shifted the Ca2+ dose-response curves (DRCs) to right similar to verapamil. In isolated guinea-pig tracheal preparations, it caused inhibition of carbachol and K+-induced bronchoconstriction at 0.1-1.0 mg/ml as well as shifted the dose-response curves (DRCs) of carbachol and histamine to the right with suppression of maximum response suggestive of non-specific bronchodilator effect mediated possibly through CCB. Pretreatment of rats with CSE (500 mg/kg orally for 2 days at 12 h intervals) prevented paracetamol (640 mg/kg) and CCl4 (150 ml/kg)-induced rise in serum alkaline phosphatase (ALP) and aminotransferases (AST and ALT). The same dose of CSE was able to prevent the CCl4-induced prolongation in pentobarbital-induced sleeping time in mice confirming its hepatoprotectivity. These results indicate the presence of calcium antagonist(s) in Carum copticum seeds and thus provides sound mechanistic basis for some of their folkloric uses.

Enhancement of acetaminophen cytotoxicity in selenium-binding protein-overexpressed COS-1 cells

The role of selenium-binding protein (SeBP), which has a high ability to associate with acetaminophen (AAP), on the cytotoxicity of AAP was studied. To clarify this issue, we examined the cytotoxic effect of AAP using COS cells stably expressing SeBP. Expression of SeBP enhanced the susceptibility of the cells to AAP-induced cytotoxicity. Several clones of SeBP-expressed COS cells were obtained, and they exhibited different degrees of susceptibility toward AAP. It was found that there is an inverse correlation between the expression level and the cell viability (r=-0.872). On the other hand, no increase in toxicity was observed in the SeBP-expressed cells treated with N-acetyl-p-quinone imine (NAPQI), which is an active metabolite of AAP. These results show that SeBP is an important factor in AAP hepatotoxicity. Moreover, our data suggest that the toxic mechanism of AAP differs from that of NAPQI.

Involvement of mitochondrial permeability transition in acetaminophen-induced liver injury in mice

BACKGROUND/AIMS

Although mitochondria have been demonstrated as primary targets in acetaminophen hepatotoxicity, the mechanism for mitochondria-mediated toxicity has not been defined. We examined the role of mitochondrial permeability transition (MPT) in the acetaminophen-induced liver injury.

METHODS

Male CD-1 mice were given intraperitoneally acetaminophen (350 mg/kg) without or with cyclosporin A (50 mg/kg), a specific inhibitor of MPT. Serum alanine aminotransferase (ALT), a marker of liver injury, and other biochemical parameters were determined.

RESULTS

Acetaminophen-induced ALT leakage was attenuated by co-administration of cyclosporin A. Cyclosporin A did not affect acetaminophen-induced early decrease in hepatic reduced glutathione (GSH) contents, indicating lack of the effect on the metabolic activation. Acetaminophen-induced decrease in mitochondrial GSH and ATP contents, and cytosolic leakage of cytochrome c were attenuated by cyclosporin A, suggesting that mitochondrial oxidative stress and ATP depletion resulting from MPT are principle mechanisms involved in acetaminophen-induced liver injury. Mitochondrial swelling by calcium was exacerbated in the mitochondria isolated from the acetaminophen-treated mice. In vitro exposure of intact mitochondria to N-acetyl-p-benzoquinone imine (NAPQI) with calcium caused mitochondrial swelling.

CONCLUSIONS

The present data indicate that the MPT is the principal mechanism in the acetaminophen-induced liver injury and NAPQI is a candidate to open the transition pore.

Acetaminophen alters microsomal ryanodine Ca2+ channel in HepG2 cells overexpressing CYP2E1

Acetaminophen hepatotoxicity is mediated by an initial metabolic activation and covalent binding of drug metabolites to liver proteins. Acetaminophen metabolites have been shown to affect rat liver microsomal Ca2+ stores, but the mechanism is not well understood. The aim of the current work was to find out if the metabolism of acetaminophen by CYP2E1 affects ryanodine-sensitive Ca2+ stores in the endoplasmic reticulum of transduced HepG2 cells. Five millimoles acetaminophen decreased proliferation of CYP2E1-overexpressing HepG2 cells, increased cytosolic Ca2+ levels and produced significant cytotoxicity, while only little, mostly anti-proliferative effects were found in HepG2 cells lacking CYP2E1. CYP2E1 inhibitor-4-methylpyrazole decreased drug cytotoxicity in transduced cells and normalized elevated Ca2+ levels. Acetaminophen cytotoxicity was significantly higher in CYP2E1 expressing cells with depleted glutathione. In the cells engineered to overexpress CYP2E1, an increased [3H]ryanodine affinity (by 45%) and increased ligand maximal binding to ryanodine receptors (by 64%) was observed, most probably due to increased association rate of [3H]ryanodine. Ca2+ loading was decreased by about 53% in microsomal fractions isolated from transduced cells treated with acetaminophen and by 92% in glutathione depleted transfected cells treated with the drug. Ca2+/Mg2+-ATPase activity was unchanged in all microsomal fractions. Such effects were not observed in cells lacking CYP2E1. Our results confirm significant role of CYP2E1 in metabolic activation of acetaminophen and indicate that ryanodine receptors located in the liver endoplasmic reticulum are sensitive targets for acetaminophen metabolites.

Studies on the protective effects of caffeic acid and quercetin on chemical-induced hepatotoxicity in rodents

Caffeic acid and quercetin, the well-known phenolic compounds widely present in the plant kingdom, were investigated for their possible protective effects against paracetamol and CCl4-induced hepatic damage. Paracetamol at the oral dose of 1 g/kg produced 100% mortality in mice while pretreatment of separate groups of animals with caffeic acid (6 mg/kg) and quercetin (10 mg/kg) reduced the death rate to 20% and 30%, respectively. Oral administration of sub-lethal dose of paracetamol (640 mg/kg) produced liver damage in rats as manifested by the significant (P<0.01) rise in serum levels of aminotransferases (aspartate transaminase (AST) and alanine transaminase (ALT)) compared to respective control values. The serum enzyme values were significantly (P<0.01) lowered on pretreatment of rats with either caffeic acid (6 mg/kg) or quercetin (10 mg/kg). Similarly, the hepatotoxic dose of CCl4 (1.5 ml/kg; orally) also raised significantly (P<0.05) the serum AST and ALT levels as compared to control values. The same dose of the caffeic acid and quercetin was able to prevent CCl4-induced rise in serum enzymes. Caffeic acid and quercetin also prevented the CCl4-induced prolongation in pentobarbital sleeping time confirming their hepatoprotectivity. These results indicate that caffeic acid and quercetin exhibited hepatoprotective activity possibly through multiple mechanisms.

Characterization of the Acetaminophen-Induced Degradation of Cytochrome P450-3A4 and the Proteolytic Pathway

It has been shown that large doses of acetaminophen can result in increased degradation of the hepatic cytochrome P450 (CYP) enzymes in vivo; however, the proteolytic pathways have not been identified. We found that incubating transfected HepG2 cells that express CYP3A4 or a reconstituted microsomal model containing human liver microsomes and cytosol, high concentrations of acetaminophen could induce a dose- and time-dependent degradation of CYP3A4. In the microsomal model the degradation could be blocked and augmented by the presence of catalase and superoxide dismutase, respectively. Tocopherol could also protect against the acetaminophen-induced degradation. However, lipid peroxidation assays showed no significant increases in lipid peroxidation products nor was there any protection by propyl gallate. Protease and proteasome inhibitors showed that the proteolytic process was mainly (85%) mediated by the lysosomal pathway, whereas a minor portion (15%) of the degradation was mediated by the proteasomal pathway. Both pepstatin A and anti-cathepsin D neutralizing antibody decreased acetaminophen-induced degradation of CYP3A4 in microsomal model systems. Pepstatin A also blocked the acetaminophen-induced degradation of the CYP3A4 in a transfected HepG2 cell line. Incubating the 3A4 cells in the presence of acetaminophen also increased cathepsin D content and activity. The lysosomal pathway, mainly mediated by cathepsin D, appears to be the major proteolytic pathway involved in the degradation of the P450 enzymes induced by toxic doses of acetaminophen.

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.

Mangafodipir prevents liver injury induced by acetaminophen in the mouse

BACKGROUND/AIMS

Acute liver failure (ALF), characterized by massive hepatocyte necrosis, is often caused by drug poisoning, particularly with acetaminophen (APAP). Hepatocyte necrosis is consecutive to glutathione depletion by NAPQI, a metabolite of APAP, and to mitochondrial damages caused by reactive oxygen species (ROS) overproduction. Considering the structure of Mangafodipir, a contrast agent currently used in magnetic resonance imaging of the liver, we hypothesized that this molecule could exert an antioxidant activity and be possibly used as a treatment of APAP-induced ALF.

METHODS/RESULTS

Mangafodipir is endowed with superoxide dismutase, catalase, and glutathione reductase activities. It can inhibit ROS production by hepatocytes in culture, and protect those cells from oxidative stresses induced by exposure to xanthine oxidase, H(2)O(2), or UV light. Moreover, preventive or curative administration of Mangafodipir to mice with APAP-induced ALF significantly increases survival rates, and abrogates aspartate aminotransferase elevation and histological damage.

CONCLUSIONS

Those data point out the potential interest of Mangafodipir in the treatment of toxic ALF in humans.

Potentiation of the antimalarial action of chloroquine in rodent malaria by drugs known to reduce cellular glutathione levels

Ferriprotoporphyrin IX (FP) is released inside the food vacuole of the malaria parasite during the digestion of host cell hemoglobin. FP is detoxified by its biomineralization to hemozoin. This process is effectively inhibited by 4-aminoquinolines. As a result FP accumulates in the membrane fraction and associates with enzymes of infected cells in parallel with parasite killing. Free FP is degraded by reduced glutathione (GSH). This degradation is inhibited by chloroquine (CQ) and amodiaquine (AQ) but not by quinine (Q) or mefloquine (MQ). Increased GSH levels in Plasmodium falciparum-infected cells confer resistance to CQ and vice versa, and sensitize CQ-resistant Plasmodium berghei by inhibiting the synthesis of glutathione. Some drugs are known to reduce GSH in body tissues when used in excess, either due to their pro-oxidant activity or their ability to form conjugates with GSH. We show that acetaminophen, indomethacin and disulfiram were able to potentiate the antimalarial action of sub-curative doses of CQ and AQ in P. berghei- or Plasmodium vinckei petteri-infected mice, but not that of Q and MQ. In contrast, N-acetyl-cysteine which is expected to increase the cellular levels of GSH, antagonized the action of CQ. Although these results imply that alteration in GSH are involved, measurement of total glutathione either in uninfected or P. berghei-infected mice, treated with these drugs did not reveal major changes. In conclusion, experimental evidences provided in this study suggest that some off the counter drugs can be used in combination with some antimalarials to which the parasite has become resistant.

Involvement of mitochondria in acetaminophen-induced apoptosis and hepatic injury: roles of cytochrome c, Bax, Bid, and caspases

The role of apoptosis in acetaminophen (AAP)-induced hepatic injury was investigated. Six hours after AAP administration to BALB/c mice, a significant loss of hepatic mitochondrial cytochrome c was observed that was similar in extent to the loss observed after in vivo activation of CD95 by antibody treatment. AAP-induced loss of mitochondrial cytochrome c coincided with the appearance in the cytosol of a fragment corresponding to truncated Bid (tBid). At the same time, tBid became detectable in the mitochondrial fraction, and concomitantly, Bax was found translocated to mitochondria. However, AAP failed to activate the execution caspases 3 and 7 as evidenced by a lack of procaspase processing and the absence of an increase in caspase-3-like activity. In contrast, the administration of the pan-inhibitor of caspases, benzyloxycarbonyl-Val-Ala-DL-Asp-fluoromethylketone (but not its analogue benzyloxycarbonyl-Phe-Ala-fluoromethylketone) prevented the development of liver injury by AAP and the appearance of apoptotic parenchymal cells. This correlated with the inhibition of the processing of Bid to tBid. The caspase inhibitor failed to prevent both the redistribution of Bax to the mitochondria and the loss of cytochrome c. In conclusion, apoptosis is an important causal event in the initiation of the hepatic injury inflicted by AAP. However, as suggested by the lack of activation of the main execution caspases, apoptosis is not properly executed and degenerates into necrosis.

Acetaminophen intoxication and length of treatment: how long is long enough?

The currently recommended dosing scheme for treating acetaminophen overdose in the United States consists of a loading dose of oral N-acetylcysteine 140 mg/kg, followed by 70 mg/kg every 4 hours for 17 doses, for a total of 72 hours of oral N-acetylcysteine therapy. This protocol has been both effective and safe. We critically evaluated the evidence that supports reducing the course of N-acetylcysteine therapy from 72 hours to 24 or 36 hours. This shorter regimen offers important benefits for both the patient and the patient's family, such as increased drug tolerability and reduced hospital stay. Patients who intentionally ingested acetaminophen with harmful intent could receive appropriate psychosocial treatment more quickly. In addition, shorter courses of N-acetylcysteine therapy have positive financial ramifications by reducing the hospital stay by 1 or 2 days. Clearly, a shorter treatment regimen would not be appropriate for all patients, particularly those who seek treatment late (> 24 hrs after ingestion) and those with evidence of organ toxicity. In order to provide the necessary evidence to support a change in accepted clinical practice, further investigation on the safety and efficacy of a shorter N-acetylcysteine regimen should be conducted by clinical researchers in a controlled manner.

Oxidant-induced cell death in renal epithelial cells: differential effects of inorganic and organic hydroperoxides

This study was undertaken in order to examine the roles of lipid peroxidation and poly (ADP-ribose) polymerase (PARP) activation in oxidant-induced renal cell death. Opossum kidney cell cultures were used as the renal epithelial cell model, and an inorganic hydroperoxide H2O2 and an organic hydroperoxide t-butylhydroperoxide were employed as model oxidants. Cell death by both oxidants could be prevented by thiols (dithiothreitol and glutathione), iron chelators (deferoxamine and phenanthroline), and hydroxyl radical scavengers (dimethylthiourea and pyruvate). Phenolic antioxidants N,N'-diphenyl-p-phenylenediamine (DPPD) and butylated hydroxyanisole had no effect on the H2O2-induced cell death. However, the t-butylhydroperoxide-induced cell death was effectively prevented by these antioxidants. The PARP inhibitor 3-aminobenzamide prevented the cell death induced by H2O2, but not cell death by t-butylhydroperoxide. The PARP activity was increased in cells exposed to H2O2 but not t-butylhydroperoxide. Unlike in opossum kidney cells, in rabbit renal cortical slices both oxidants H2O2 and t-butylhydroperoxide induced cell death through a lipid peroxidation-dependent and PARP-independent mechanism. Effects of DPPD and 3-aminobenzamide on H2O2-induced cell death in primary cultured rabbit proximal tubular cells were similar to those in opossum kidney cells. These results indicate that 1) the H2O2-induced cell death in cultured renal epithelial cells is associated with PARP activation but not lipid peroxidation, whereas the t-butylhydroperoxide-induced cell death is mediated by lipid peroxidation, and 2) the role of lipid peroxidation in H2O2 cytotoxicity may be different between freshly isolated renal tubular cells and cultured renal epithelial cells.

Protective effect of moutan cortex extract on acetaminophen-induced cytotoxicity in human Chang liver cells

The aim of this study was to investigate the effect of Moutan Cortex on acetaminophen (AAP)-induced toxicity in human Chang liver cells. Cells were incubated with AAP (0-30 mM) to evaluate the drug's ability to reduce cytoviability. For the cells treated with 10, 20 and 30 mM AAP, LDH leakage was 39.8%, 49.0% and 57.6%, respectively. Administration of Moutan Cortex reduced cytotoxicity in a dose-dependent manner. Glutathione (GSH) concentration in human liver cells decreased significantly after exposure to 20 (p<0.05) and 30 mM (p<0.01) AAP, and increased (p<0.05) if incubated with AAP and Moutan Cortex. The ability of AAP to inhibit mitochondrial function and its counteraction by Moutan Cortex was also evaluated. Moutan Cortex showed dose-dependent increases in MTT metabolism and ATP levels in AAP-treated cells. The DNA content of AAP-treated cells increased with the treatment of Moutan Cortex. These observations demonstrate that Moutan Cortex may significantly attenuate AAP-induced toxicity. It can be considered a cytoprotective agent in this in vitro model of drug toxicity.

Cell culture model for acetaminophen-induced hepatocyte death in vivo

Overdose of the popular, and relatively safe, analgesic acetaminophen (N-acetyl-p-aminophenol, APAP, paracetamol) can produce a fatal centrilobular liver injury. APAP-induced cell death was investigated in a differentiated, transforming growth factor alpha (TGFalpha)-overexpressing, hepatocyte cell line and found to occur at concentrations, and over time frames, relevant to clinical overdose situations. Coordinated multiorganellar collapse was evident during APAP-induced cytotoxicity with widespread, yet selective, protein degradation events in vitro. Cellular proteasomal activity was inhibited with APAP treatment but not with the comparatively nonhepatotoxic APAP regioisomer, N-acetyl-m-aminophenol (AMAP). Low concentrations of the proteasome-directed inhibitor MG132 (N-carbobenzoxyl-Leu-Leu-Leucinal) increased chromatin condensation and cellular stress responses preferentially in AMAP-treated cultures, suggesting a contribution of the proteasome in APAP- but not AMAP-mediated cell death. APAP-specific alterations to mitochondria were observed morphologically with evidence of mitochondrial proliferation in vitro. Biochemical alterations to cellular proteolytic events were also found in vivo, including APAP- or AMAP-mediated inhibition of caspase-3 processing. These results indicate that, although retaining some attributes of apoptosis, both APAP- and AMAP-mediated cell death have additional distinctive features consistent with longer term necrosis.

Tubifex: a sensitive model for UV-B-induced phototoxicity

The natural increase of UV-B radiation levels due to depletion of the ozone layer in the atmosphere may impose additional stress for the survival of zooplanktons which serve as a major constituent of the aquatic food chain. To study the adverse effects of UV-B radiation on the aquatic biomass, studies were conducted using the aquatic organism Tubifex as a model, as UV-B radiation is known to penetrate into the natural waters. UV-B radiation induced mortality in tubifex and the production of activated oxygen species by these organisms. Alterations in DNA, RNA, protein, glutathione (GSH), hydrogen peroxide H(2)O(2), thiobarbituric acid-reactive substance (TBA-RS), ATPase, AChE, GST, and LDH activities in Tubifex at various doses (0-2.0 J) of UV-B radiation were found. LC(50) value for UV-B-induced mortality of Tubifex was 0.80+/-0.15 J and the threshold dose was 0.35+/-0.05 J; mortality began within 3h postirradiation. UV-B dose-dependent production of singlet oxygen, superoxide anion, and hydroxyl radicals by Tubifex was observed. DNA, RNA, protein, and GSH contents were found to decrease significantly (P<0.001) while H(2)O(2) and TBA-RS increased (P<0.01) under the influence of UV-B radiation. The activities of ATpase, AChE, and GST enzymes were inhibited (P<0.01) and LDH activity was significantly increased (P<0.001) in Tubifex exposed to UV-B radiation. The results suggest that an increase in UV-B radiation alters several biochemical processes, leading to the mortality of the organism. Tubifex could be useful as a sensitive alternate model for studying UV-B-induced phototoxicity and possible mechanisms of action.

Thiol antioxidants inhibit the adjuvant effects of aerosolized diesel exhaust particles in a murine model for ovalbumin sensitization

Although several epidemiological studies indicate a correlation between exposure to ambient particulate matter and adverse health effects in humans, there is still a fundamental lack of understanding of the mechanisms involved. We set out to test the hypothesis that reactive oxygen species are involved in the adjuvant effects of diesel exhaust particles (DEP) in a murine OVA sensitization model. First, we tested six different antioxidants, N-acetylcysteine (NAC), bucillamine (BUC), silibinin, luteolin, trolox (vitamin E), and ascorbic acid, for their ability to interfere in DEP-mediated oxidative stress in vitro. Of the six agents tested, only the thiol antioxidants, BUC and NAC, were effective at preventing a decrease in intracellular reduced glutathione:glutathione disulfide ratios, protecting cells from protein and lipid oxidation, and preventing heme oxygenase 1 expression. Therefore, we selected the thiol antioxidants for testing in the murine OVA inhalation sensitization model. Our data demonstrate that NAC and BUC effectively inhibited the adjuvant effects of DEP in the induction of OVA-specific IgE and IgG1 production. Furthermore, NAC and BUC prevented the generation of lipid peroxidation and protein oxidation in the lungs of OVA- plus DEP-exposed animals. These findings indicate that NAC and BUC are capable of preventing the adjuvant effects of inhaled DEP and suggest that oxidative stress is a key mechanistic component in the adjuvant effect of DEP. Antioxidant treatment strategies may therefore serve to alleviate allergic inflammation and may provide a rational basis for treating the contribution of particulate matter to asthmatic disease.

Acetaminophen-induced hepatotoxicity in mice lacking inducible nitric oxide synthase activity

We recently reported that nitrotyrosine and acetaminophen (APAP)-cysteine protein adducts colocalize in the hepatic centrilobular cells following a toxic dose of APAP to mice. Whereas APAP-adducts are formed by reaction of the metabolite N-acetyl-p-benzoquinone imine with cysteine, nitrotyrosine residues are formed by reaction of tyrosine with peroxynitrite. Peroxynitrite is formed from nitric oxide (NO) and superoxide. This manuscript examines APAP (300 mg/kg) hepatotoxicity in mice lacking inducible nitric oxide synthase activity (NOS2 null or knockout mice; C57BL/6-Nos2(tm1Lau)) and in the wildtype mice. In a time course the ALT levels in the exposed NOS2 null mice were approximately 50% of the wildtype mice; however, histological examination of liver sections indicated similar levels of centrilobular hepatic necrosis in both wild-type and NOS2 null mice. Serum nitrate plus nitrite levels (NO synthesis) were identical in saline-treated NOS2 null and wild-type mice (53 +/- 2 microM). APAP increased NO synthesis in wild-type mice only. The increases paralleled the increases in ALT levels with peak levels of serum nitrate plus nitrite at 6 h (168 +/- 27 microM). In wild-type mice hepatic tyrosine nitration was greatly increased relative to saline treated controls. Tyrosine nitration increased in NOS2 null mice also, but the increase was much less. APAP increased hepatic malonaldehyde levels (lipid peroxidation) in NOS2 null mice only. The results suggest the presence of multiple pathways to APAP-mediated hepatic necrosis, one via nitrotyrosine, as in the wild-type mice, and another that is not dependent upon inducible nitric oxide synthase activity, but which may involve increased superoxide.

In vitro hepatotoxicity of alachlor and its by-products

During water treatment, potentially hazardous chemical by-products may be formed. Alachlor (2-chloro-N-(2, 6-diethylphenyl)-N-(methoxymethyl) acetamide) is a widely used pre-emergence herbicide. The present study investigated the toxicity of alachlor and its disinfection by-products on freshly isolated rat hepatocytes. Hepatocytes were harvested by a collagenase perfusion technique and were exposed to different concentrations of alachlor and its by-products for up to 2 h. Cell viability, the leakage of aspartate transaminase (AST) and alanine transaminase (ALT) and glutathione (GSH) depletion were determined throughout the incubation period. The cell viability of the hepatocytes exposed to 100 microg ml(-1) alachlor was decreased by 20% compared with the control after 60 min of incubation. At the same concentration of alachlor the leakage of ALT and AST was increased by 56% and 45%, respectively. Cell viability of the hepatocytes was decreased upon exposure to 2-chloro-N-(3-chloro-2,6-diethylphenyl)-N-(methoxymethyl) acetamide (CCDMA) and 2-chloro-N-(3-chloro-2,6-diethylphenyl) acetamide (CCDA)--the by-products of alachlor and chlorine--after 60 min of exposure. At 100 microg ml(-1) CCDMA the AST leakage was increased significantly (73%) after 30 min of incubation. The reaction mixture of alachlor (100 microg ml(-1)) and chlorine dioxide (1 ppm) caused significant increases in cell loss and ALT and AST levels by 22%, 40% and 34%, respectively, as early as 15 min incubation. Alachlor (100 and 200 microg ml(-1)) caused significant decreases in GSH contents (62%) in isolated hepatocytes. The reaction mixture of alachlor and chlorine dioxide led to significant glutathione depletion (44%) after 60 min of incubation. The by-products of alachlor and chlorine--CCDMA and CCDA--depleted GSH almost completely (93%). This investigation suggested that the by-products formed from the reaction of alachlor and chlorine decreased GSH and increased the leakage of liver enzymes, especially AST.

Establishment of the transformants expressing human cytochrome P450 subtypes in HepG2, and their applications on drug metabolism and toxicology

Transformants with stable expression of a series of human cytochrome P450 (CYP) subtypes in the human hepatic cell line, HepG2, were established. These transformants are designated Hepc/1A1.4, Hepc/1A2.9, Hepc/2A6L.14, Hepc/2B6.68, Hepc/2C8.46, Hepc/2C9.1, Hepc/2C19.12, Hepc/2D6.39, Hepc/2E1.3-8 and Hepc/3A4.2-30, which stably expressed human CYP1A1, CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4, respectively. The expression of the CYP subtypes in the transformants was confirmed by both determination of enzyme activities and the reverse transcriptase polymerase chain reaction (RT-PCR) procedure. The apparent K(m) values of the expressed CYP subtypes for their specific substrates were close to those of human liver microsomes. In addition to their CYP activities, these transformants retained glucuronide- and sulfate-conjugating activities. Furthermore, the activities of CYP2C9, CYP2D6 and CYP3A4 were inhibited by their specific inhibitors. The cytotoxicity of acetaminophen (APAP), cyclophosphamide (CPA) and benz[a]anthracene (BA) were analyzed by CYP-expressing transformants. The cytotoxicity depended on the expression of CYP subtypes and increased in a dose-dependent manner. These results show the metabolic activation of APAP, CPA and BA by the specific CYP subtypes expressed in the transformants and demonstrate the usefulness of these transformants for in vitro metabolic and toxicological studies in human liver.

Evaluation of hepatoprotective activity of legumes

Mung bean, adzuki bean, black bean and rice bean are foods and folk medicines of Taiwan. We evaluated the effects of various water extract concentrations (100, 500 and 1000 mg/kg body wt.) and silymarin (25 mg/kg body wt. on acetaminophen-induced liver injury by measuring serum glutamate-oxalate-transaminase (sGOT) and serum glutamate-pyruvate-transaminase (sGPT) activities in rats. The results showed that the sGOT and the sGPT activities, increased by APAP, were decreased significantly (P < 0.05) through treatment with inceasing amounts up to 1000 mg/kg body wt. of the exracts. In particular, the mung bean aqueous extract showed the best hepatoprotective effect on APAP-induced hepatotoxicity. The pathological changes of liver injury caused by APAP improved by the treatment with all of the legume extracts, which were compared to silymarin as a standardized drug. In addition to these results, the extract of mung bean acted as a potential hepatoprotective agent in dietary supply.

Expression profiling of acetaminophen liver toxicity in mice using microarray technology

Drug-induced hepatotoxicity causes significant morbidity and mortality and is a major concern in drug development. This is due, in large part, to insufficient knowledge of the mechanism(s) of drug-induced liver injury. In order to address this problem, we have evaluated the modulation of gene expression within the livers of mice treated with a hepatotoxic dose of acetaminophen (APAP) using high-density oligonucleotide microarrays capable of determining the expression profile of >11,000 genes and expressed sequence tags (ESTs). Significant alterations in gene expression, both positive and negative, were noted within the livers of APAP-treated mice. APAP-induced toxicity affected numerous aspects of liver physiology causing, for instance, >twofold increased expression of genes that encode for growth arrest and cell cycle regulatory proteins, stress-induced proteins, the transcription factor LRG-21, suppressor of cytokine signaling (SOCS)-2-protein, and plasminogen activator inhibitor-1 (PAI-1). A number of these and other genes and ESTs were detectable within the liver only after APAP treatment suggesting their potential importance in propagating or preventing further toxicity. These data provide new directions for mechanistic studies that may lead to a better understanding of the molecular basis of drug-induced liver injury and, ultimately, to a more rational design of safer drugs.

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.

Antioxidant and hepatoprotective effects of punicalagin and punicalin on acetaminophen-induced liver damage in rats

Punicalagin and punicalin were isolated from the leaves of Terminalia catappa L., a Combretaceous plant distributed throughout tropical and subtropical beaches, which is used for the treatment of dermatitis and hepatitis. Our previous studies showed that both of these compounds exert antioxidative activity. In this study, the antihepatotoxic activity of punicalagin and punicalin on acetaminophen-induced toxicity in the rat liver was evaluated. After evaluating the changes of several biochemical functions in serum, the levels of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) were increased by acetaminophen administration and reduced by punicalagin and punicalin. Histological changes around the hepatic central vein and oxidative damage induced by acetaminophen were also recovered by both compounds. The data show that both punicalagin and punicalin exert antihepatotoxic activity, but treatment with larger doses enhanced liver damage. These results suggest that even if punicalagin and punicalin have antioxidant activity at small doses, treatment with larger doses will possibly induce some cell toxicities.

Antioxidant and hepatoprotective effects of Anoectochilus formosanus and Gynostemma pentaphyllum

Anoectochilus formosanus Hay. and Gynostemma pentaphyllum Makino are popular folk medicines that have been used for treating hepatitis, hypertension and cancer in Taiwan. Our previous studies showed that these crude drugs exert antiinflammatory activity and hepatoprotective activity against CC14-induced liver damage. In this study, the antioxidant effect of these crude drugs and their hepatoprotective activity on acetaminophen-induced liver injury in rat was evaluated. Our results suggest that A. formosanus and G. pentaphyllum do have antioxidant effects. On acetaminophen-intoxicated model, the increased levels of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) by acetaminophen administration were reduced by treatment with these two herbs. In histological observation, gross necrosis in the centribular area, sinusoidal congestion, infiltration of the lymphocytes and Kupffer cells around the hepatic central vein, and loss of cell boundaries and ballooning degeneration were reduced with herbal treatment. However, the effect of A. formosanus and G. pentaphyllum is biphasic. Methanol extract (100 and 300 mg/kg) and water extract (300 and 500 mg/kg) of A formosanus and water extract (100, 300 and 500 mg/kg) of G. pentaphyllum enhanced the recovery of liver injury while treatment with 500 mg/kg of A. formosanus methanol extract resulted in serious hepatic injury.

CYP2E1 overexpression in HepG2 cells induces glutathione synthesis by transcriptional activation of gamma-glutamylcysteine synthetase

Induction of CYP2E1 (cytochrome P450 2E1) by ethanol appears to be one of the central pathways by which ethanol generates a state of oxidative stress. CYP2E1 is a loosely coupled enzyme; formation of reactive oxygen species occurs even in the absence of added substrate. GSH is critical for preserving the proper cellular redox balance and for its role as a cellular protectant. Since cells must maintain optimal GSH levels to cope with a variety of stresses, the goal of this study was to characterize the GSH homeostasis in human hepatocarcinoma cells (HepG2) that overexpress CYP2E1. This study was prompted by the finding that toxicity in CYP2E1-overexpressing cells was markedly enhanced after GSH depletion by buthionine sulfoximine treatment. CYP2E1-overexpressing cells showed a 40-50% increase in intracellular H(2)O(2); a 30% increase in total GSH levels; a 50% increase in the GSH synthesis rate; and a 2-fold increase in gamma-glutamylcysteine synthetase heavy subunit (GCS-HS) mRNA, the rate-limiting enzyme in GSH synthesis. This GCS-HS mRNA increase was due to increased synthesis since nuclear run-on assays showed increased transcription in CYP2E1-expressing cells, and the GCS-HS mRNA decay after actinomycin D treatment was similar in CYP2E1-expressing cells and empty vector-transfected cells. The facts that treatment with GSH ethyl ester almost completely prevented the increase in GCS-HS mRNA and decreased H(2)O(2) levels and that transient transfection with catalase (but not manganese-superoxide dismutase) produced a decrease in GCS-HS mRNA only in CYP2E1-expressing cells suggest a possible role for H(2)O(2) in the induction of GCS-HS gene transcription. In contrast to results with HepG2 cells expressing CYP2E1, no increase in GCS-HS mRNA was found with a HepG2 cell line engineered to express human cytochrome P450 3A4. In summary, CYP2E1 overexpression in HepG2 cells up-regulates the levels of reduced GSH by transcriptional activation of GCS-HS; this may reflect an adaptive mechanism to remove CYP2E1-derived oxidants such as H(2)O(2).

Effect of acetaminophen on Na(+), K(+) ATPase and alkaline phosphatase on plasma membranes of renal proximal tubules

In previous work we reported that 1 h after acetaminophen (APAP) administration, tubular function remained at control values, while 16 h later a significant deterioration of tubular function was observed. The aim of the present work was to study if APAP induces its renal toxic effects by altering the normal activity of key tubular plasma membrane enzymes. We analyzed the effects of a nephrotoxic dose of APAP (1000 mg/kg b.wt., i.p.) on the activities of the brush-border membrane (BBM) enzyme, alkaline phosphatase, and the basolateral membrane (BLM) enzyme Na(+), K(+) ATPase 1 h (APAP(1h)) and 16 h (APAP(16h)) after dosing. Na(+), K(+) ATPase abundance in homogenates and each membrane domain were analyzed by Western blot. Cortical adenosine 5' triphosphate (ATP) content was also evaluated. At each time studied, APAP promoted a diminution of alkaline phosphatase in BBM. Na(+), K(+) ATPase activity in BLM showed a biphasic response to APAP. One hour after APAP administration it was significantly increased, but it was decreased 16 h after dosing. Na(+), K(+) ATPase protein abundance was elevated in homogenates, BLM, and BBM after 1 h of APAP dosing. After 16 h, Na(+), K(+) ATPase abundance was increased in homogenates, while in BLM it was decreased. No differences were observed in cortical ATP content in each time studied. Our present results could contribute to the understanding of the molecular basis of the previously reported time course alteration in the fractional excretion of sodium promoted by a nephrotoxic dose of APAP.

Acetaminophen intoxication during treatment: what you don't know can hurt you

For over two decades, pediatricians have been made aware of the potential risk associated with the acute ingestion of large single and/or multiple doses of acetaminophen (APAP). Clearly, APAP-induced hepatotoxicity remains as a recognized medical emergency which, when treated promptly with appropriate gastrointestinal decontamination and when indicated, with the antidote N-acetylcysteine, has a uniformly good clinical outcome. Recently, the hepatotoxic potential associated with "therapeutic" APAP administration has been brought to the attention of the pediatric community. This review explores the issue of APAP toxicity with therapeutic intent by examining both the clinical literature and also, relevant information concerning the basic pharmacology and toxicology of this old and widely used nonprescription drug. A "risk profile" is developed with regard to factors that may predispose infants and children to this iatrogenic form of toxicity so that the awareness of physicians and other caregivers (including parents) can be heightened and preventative education administered. As is true for most all potentially beneficial medicines used in pediatrics, awareness of the actual amount of drug received from all sources and caution to not exceed the age-appropriate dosing guidelines (i.e., both amount and duration) contained in the approved labeling for all products containing APAP will insure safe and effective therapy.

The aromatic amine carcinogens o-toluidine and o-anisidine induce free radicals and intrachromosomal recombination in Saccharomyces cerevisiae

Aniline-based aromatic amine carcinogens are poorly detected in short-term mutagenicity assays such as the Salmonella reverse mutation (Ames) assay. More information on the mechanism of toxicity of such Salmonella-negative carcinogens is needed. Aniline and o-toluidine are negative in the Ames assay, but induce deletions (DEL) due to intrachromosomal recombination in Saccharomyces cerevisiae with an apparent threshold. We show here that the DEL assay also detects the genotoxic activity of another aromatic amine carcinogen, o-anisidine, which is also negative in the Salmonella assay. We also show that the DEL assay distinguishes between o-anisidine and its non-carcinogenic structural analog 2, 4-dimethoxyaniline. We have investigated whether the ability of the DEL assay to detect the carcinogens and to distinguish between the carcinogen/non-carcinogen pair is linked to rises in intracellular free radical species following exposure to the carcinogens. Toxicity induced by all three compounds was reduced in the presence of the free radical scavenger and antioxidant N-acetyl cysteine, recombination induced by o-anisidine and o-toluidine was also reduced by N-acetyl cysteine. All three compounds induced oxidation of the free radical-sensitive reporter compound dichlorofluorescin diacetate. Superoxide dismutase-deficient strains, however, were hypersensitive to cytotoxicity induced by o-toluidine and o-anisidine but not by the non-carcinogen 2,4-dimethoxyaniline, indicating a different potential for generating superoxide radical between the carcinogens and the non-carcinogen analog. The results indicate that the yeast DEL assay is a useful tool for investigating the genotoxic activity of aromatic amine carcinogens.

Resistance of three immortalized human hepatocyte cell lines to acetaminophen and N-acetyl-p-benzoquinoneimine toxicity

BACKGROUND/AIMS

Acetaminophen toxicity in hepatocytes is attributed to generation of the toxic metabolite N-acetyl-p-benzoquinoneimine, leading to depletion of intracellular glutathione, alteration of redox potential and ultimately, cellular necrosis. We aimed to determine the effect of acetaminophen and N-acetyl-p-benzoquinoneimine on three human hepatocyte cell lines HH25, HH29 and HHY41, and for comparison, on primary rat hepatocytes, a cell type that is relatively resistant to acetaminophen-induced toxicity.

METHODS

We investigated the effect of incubation of rat hepatocytes and 3 hepatocyte cell lines with acetaminophen or N-acetyl-p-benzoquinoneimine on LDH release, glutathione status, mitochondrial function, CYP1A activity, albumin synthesis and DNA content.

RESULTS

We demonstrated that HH25, HH29 and HHY41 are resistant to the toxic effects of acetaminophen under conditions that induce cytotoxicity in rat primary hepatocytes, as indicated by maintenance of glutathione levels and basal LDH release. Incubation with N-acetyl-p-benzoquinoneimine caused a dose-dependent cytotoxicity in rat hepatocytes. Under comparable conditions N-acetyl-p-benzoquinoneimine had no effect on any of the hepatocyte cell lines. Nevertheless, when culturing the cells for a further 48 h, a decrease in glutathione levels, albumin synthesis, CYP1A activity, DNA content and mitochondrial function was apparent.

CONCLUSION

HH25, HH29 and HHY41 cells are highly resistant to acetaminophen and N-acetyl-p-benzoquinoneimine-induced toxicity. They tolerate a much higher concentration of both toxins for a longer period of time compared to rat primary hepatocytes. These results are of relevance in the use of these cell lines to investigate acetaminophen hepatotoxicity, and may be of importance in the choice of cells for use in bioartificial liver support systems.

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.

Repeat exposure to incremental doses of acetaminophen provides protection against acetaminophen-induced lethality in mice: an explanation for high acetaminophen dosage in humans without hepatic injury

In studies designed to simulate a clinical observation in which an individual became tolerant to normally lethal doses of acetaminophen (APAP), mice were pretreated with increasing doses of APAP for 8 days and challenged on day 9 with normally supralethal doses of APAP. These animals developed minimal hepatotoxicity after a challenge dose with a fourfold increase in LD50 to 1,350 mg/kg. The pretreatment regimen resulted in hepatic changes including: centrilobular localization of 3-(cysteine-S-yl)APAP protein adducts, selective down-regulation of cytochrome P4502E1 (CYP2E1) and CYP1A2 that produced the toxic metabolite, N-acetyl-p-benzoquinone imine, higher levels of reduced glutathione (GSH), centrilobular inflammation, and a fourfold increase in hepatocellular proliferation. The protection against the lethal APAP doses afforded by pretreatment is secondary to these changes and to the associated regional shift in the bioactivation of the APAP challenge dose from centrilobular to periportal regions where CYP2E1 is not found, protective GSH is more abundant, and where cell-proliferative responses are better able to sustain repair. This shift in APAP bioactivation results in less-intense covalent binding that is more diffuse and spread uniformly throughout the hepatic lobe, most likely contributing to protection by delaying the early onset of liver injury that has been generally associated with centrilobular localization of the adducts. Intervention of APAP pretreatment-induced cell division in mice with colchicine left them resistant to a 500-mg/kg (normally lethal) dose of APAP, but unable to survive a 1,000-mg/kg APAP challenge dose. The data demonstrate multiple mechanistic components to the protection afforded by APAP pretreatment. Whereas metabolic and physiological changes not dependent on cell proliferation are adequate to protect against 500 mg/kg APAP, these changes plus a potentiated cell-proliferative response are necessary for protection against the supralethal 1,000-mg/kg APAP dose. Furthermore, the data document an uncoupling of the traditional association between covalent binding and toxicity, and suggest that the assessment of toxicity following repeated or chronic APAP exposure must consider altered drug interactions and parameters besides those historically used to assess acute APAP overdose.

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.

Lipid peroxidation induced by adriamycin in linolenic acid-loaded cultured hepatocytes

Addition of more than 10 microM of adriamycin to cultured rat hepatocytes loaded with alpha-linolenic acid (linolenic acid-loaded hepatocytes) caused marked lipid peroxidation as measured by an accumulation of malondialdehyde during a 9 hr incubation. After addition of 50 microM of adriamycin to linolenic acid-loaded hepatocytes, malondialdehyde accumulation significantly increased at 3 hr, followed by cellular reduced glutathione decrease and lactate dehydrogenase leakage after 6 hr. Inhibition of adriamycin-induced lipid peroxidation by addition of N,N'-diphenyl-p-phenylenediamine or alpha-tocopherol, both lipid radical scavengers, or deferoxamine, which is a Fe ion chelator, prevented both glutathione decrease and lactate dehydrogenase leakage, indicating that lipid peroxidation caused cellular damage to linolenic acid-loaded hepatocytes exposed to adriamycin. The effect of SKF 525-A, which is a cytochrome P450 inhibitor, on adriamycin-induced lipid peroxidation and on 7-ethoxycoumarin O-deethylase activity was determined by 6 hr incubation of linolenic acid-loaded cells. Addition of SKF 525-A suppressed adriamycin-induced lipid peroxidation comparably with its 7-ethoxy-coumarin 0-deethylase inhibitory activity. These results suggest that cytochrome P450 contributes to the one-electron bioreduction of adriamycin into its semiquinone radical in rat hepatocytes.