Acetaminophen and protein thiol modification

Dihydroceramide Desaturase Functions as an Inducer and Rectifier of Apoptosis: Effect of Retinol Derivatives, Antioxidants and Phenolic Compounds

Dihydroceramide desaturase (Degs1) catalyses the introduction of a 4,5-trans double bond into dihydroceramide to form ceramide. We show here that Degs1 is polyubiquitinated in response to retinol derivatives, phenolic compounds or anti-oxidants in HEK293T cells. The functional predominance of native versus polyubiquitinated forms of Degs1 appears to govern cytotoxicity. Therefore, 4-HPR or celecoxib appear to stimulate the de novo ceramide pathway (with the exception of C24:0 ceramide), using native Degs1, and thereby promote PARP cleavage and LC3B-I/II processing (autophagy/apoptosis). The ubiquitin-proteasomal degradation of Degs1 is positively linked to cell survival via XBP-1s and results in a concomitant increase in dihydroceramides and a decrease in C24:0 ceramide levels. However, in the case of 4-HPR or celecoxib, the native form of Degs1 functionally predominates, such that the apoptotic programme is sustained. In contrast, 4-HPA or AM404 do not produce apoptotic ceramide, using native Degs1, but do promote a rectifier function to induce ubiquitin-proteasomal degradation of Degs1 and are not cytotoxic. Therefore, Degs1 appears to function both as an 'inducer' and 'rectifier' of apoptosis in response to chemical cellular stress, the dynamic balance for which is dependent on the nature of chemical stress, thereby determining cytotoxicity. The de novo synthesis of ceramide or the ubiquitin-proteasomal degradation of Degs1 in response to anti-oxidants, retinol derivatives and phenolic compounds appear to involve sensors, and for rectifier function, this might be Degs1 itself.

TAMH: A Useful In Vitro Model for Assessing Hepatotoxic Mechanisms

In vitro models for hepatotoxicity can be useful tools to predict in vivo responses. In this review, we discuss the use of the transforming growth factor-α transgenic mouse hepatocyte (TAMH) cell line, which is an attractive model to study drug-induced liver injury due to its ability to retain a stable phenotype and express drug-metabolizing enzymes. Hepatotoxicity involves damage to the liver and is often associated with chemical exposure. Since the liver is a major site for drug metabolism, drug-induced liver injury is a serious health concern for certain agents. At the molecular level, various mechanisms may protect or harm the liver during drug-induced hepatocellular injury including signaling pathways and endogenous factors (e.g., Bcl-2, GSH, Nrf2, or MAPK). The interplay between these and other pathways in the hepatocyte can change upon drug or drug metabolite exposure leading to intracellular stress and eventually cell death and liver injury. This review focuses on mechanistic studies investigating drug-induced toxicity in the TAMH line and how alterations to hepatotoxic mechanisms in this model relate to the in vivo situation. The agents discussed herein include acetaminophen (APAP), tetrafluoroethylcysteine (TFEC), flutamide, PD0325901, lapatinib, and flupirtine.

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.

Endogenous interleukin-4 regulates glutathione synthesis following acetaminophen-induced liver injury in mice

In a recent study, we reported that interleukin (IL)-4 had a protective role against acetaminophen (APAP)-induced liver injury (AILI), although the mechanism of protection was unclear. Here, we carried out more detailed investigations and have shown that one way IL-4 may control the severity of AILI is by regulating glutathione (GSH) synthesis. In the present studies, the protective role of IL-4 in AILI was established definitively by showing that C57BL/6J mice made deficient in IL-4 genetically (IL-4(-/-)) or by depletion with an antibody, were more susceptible to AILI than mice not depleted of IL-4. The increased susceptibility of IL-4(-/-) mice was not due to elevated levels of hepatic APAP-protein adducts but was associated with a prolonged reduction in hepatic GSH that was attributed to decreased gene expression of γ-glutamylcysteine ligase (γ-GCL). Moreover, administration of recombinant IL-4 to IL-4(-/-) mice postacetaminophen treatment diminished the severity of liver injury and increased γ-GCL and GSH levels. We also report that the prolonged reduction of GSH in APAP-treated IL-4(-/-) mice appeared to contribute toward increased liver injury by causing a sustained activation of c-Jun-N-terminal kinase (JNK) since levels of phosphorylated JNK remained significantly higher in the IL-4(-/-) mice up to 24 h after APAP treatment. Overall, these results show for the first time that IL-4 has a role in regulating the synthesis of GSH in the liver under conditions of cellular stress. This mechanism appears to be responsible at least in part for the protective role of IL-4 against AILI in mice and may have a similar role not only in AILI in humans but also in pathologies of the liver caused by other drugs and etiologies.

Proteomic analysis of acetaminophen-induced changes in mitochondrial protein expression using spectral counting

Comparative proteomic analysis following treatment with acetaminophen (APAP) was performed on two different models of APAP-mediated hepatocellular injury in order to both identify common targets for adduct formation and track drug-induced changes in protein expression. Male C57BL/6 mice were used as a model for APAP-mediated liver injury in vivo, and TAMH cells were used as a model for APAP-mediated cytotoxicity in vitro. SEQUEST was unable to identify the precise location of sites of adduction following treatment with APAP in either system. However, semiquantitative analysis of the proteomic data sets using spectral counting revealed a downregulation of P450 isoforms associated with APAP bioactivation and an upregulation of proteins related to the electron transport chain by APAP compared to the control. Both mechanisms are likely compensatory in nature as decreased P450 expression is likely to attenuate toxicity associated with N-acetyl-p-quinoneimine (NAPQI) formation, whereas APAP-induced electron transport chain component upregulation may be an attempt to promote cellular bioenergetics.

Acetaminophen-associated hepatic injury: evaluation of acetaminophen protein adducts in children and adolescents with acetaminophen overdose

Acetaminophen protein adducts (APAP adducts) were quantified in 157 adolescents and children presenting at eight pediatric hospitals with the chief complaint of APAP overdose. Two of the patients required liver transplantation, whereas all the others recovered spontaneously. Peak APAP adducts correlated with peak hepatic transaminase values, time-to-treatment with N-acetylcysteine (NAC), and risk determination per the Rumack-Matthews nomogram. A population pharmacokinetic analysis (NONMEM) was performed with post hoc empiric Bayesian estimates determined for the elimination rate constants (k(e)), elimination half-lives (t(1/2)), and maximum concentration of adducts (C(max)) of the subjects. The mean (+/-SD)k(e) and half-life were 0.486 +/- 0.084 days(-1) and 1.47+/- 0.30 days, respectively, and the C(max) was 1.2 (+/-2.92) nmol/ml serum. The model-derived, predicted adduct value at 48 h (Adduct 48) correlated with adductC(max), adduct T(max), Rumack-Matthews risk determination, peak aspartate aminotransferase (AST), and peak alanine aminotransferase (ALT). The pharmacokinetics and clinical correlates of APAP adducts in pediatric and adolescent patients with APAP overdose support the need for a further examination of the role of APAP adducts as clinically relevant and specific biomarkers of APAP toxicity.

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.