Selective protein arylation by acetaminophen and 2,6-dimethylacetaminophen in cultured hepatocytes from phenobarbital-induced and uninduced mice. Relationship to cytotoxicity

Mitochondrial protein adducts formation and mitochondrial dysfunction during N-acetyl-m-aminophenol (AMAP)-induced hepatotoxicity in primary human hepatocytes

3'-Hydroxyacetanilide orN-acetyl-meta-aminophenol (AMAP) is generally regarded as a non-hepatotoxic analog of acetaminophen (APAP). Previous studies demonstrated the absence of toxicity after AMAP in mice, hamsters, primary mouse hepatocytes and several cell lines. In contrast, experiments with liver slices suggested that it may be toxic to human hepatocytes; however, the mechanism of toxicity is unclear. To explore this,we treated primary human hepatocytes (PHH) with AMAP or APAP for up to 48 h and measured several parameters to assess metabolism and injury. Although less toxic than APAP, AMAP dose-dependently triggered cell death in PHH as indicated by alanine aminotransferase (ALT) release and propidium iodide (PI) staining. Similar to APAP, AMAP also significantly depleted glutathione (GSH) in PHH and caused mitochondrial damage as indicated by glutamate dehydrogenase (GDH) release and the JC-1 assay. However, unlike APAP, AMAP treatment did not cause relevant c-jun-N-terminal kinase (JNK) activation in the cytosol or phospho-JNK translocation to mitochondria. To compare, AMAP toxicity was assessed in primary mouse hepatocytes (PMH). No cytotoxicity was observed as indicated by the lack of lactate dehydrogenase release and no PI staining. Furthermore, there was no GSH depletion or mitochondrial dysfunction after AMAP treatment in PMH. Immunoblotting for arylated proteins suggested that AMAP treatment caused extensive mitochondrial protein adduct formation in PHH but not in PMH. In conclusion, AMAP is hepatotoxic in PHH and the mechanism involves the formation of mitochondrial protein adducts and mitochondrial dysfunction.

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.

Two-dimensional database of mouse liver proteins: changes in hepatic protein levels following treatment with acetaminophen or its nontoxic regioisomer 3-acetamidophenol

Overdose of acetaminophen (APAP) causes acute hepatotoxicity in rodents and man. The mechanism underlying APAP-induced liver injury remains unclear, but experimental evidence strongly suggests that activation of APAP and subsequent formation of protein adducts are involved in hepatotoxicity. Using proteomics technologies, we constructed a two-dimensional protein database for mouse liver, comprising 256 different gene products and investigated the proteins affected after APAP-induced hepatotoxicity. Adult male mice received a single dose of APAP (100 or 300 mg/kg) or its nontoxic regioisomer 3-acetamidophenol (AMAP, 300 mg/kg). The extent of liver damage was assessed 8 h after administration by increased liver enzyme release and histopathology. Changes in the protein level were studied by comparison of the intensities of the corresponding spots on two-dimensional (2-D) gels. The expression level of about 35 of the identified proteins was modified due to treatment with APAP or AMAP. The observed changes were usually in the order of 10-50% of the control value and were more marked in the high- than in the low-dose of APAP-treated animals. Most of the changes caused by AMAP occurred in a subset of the proteins modified by APAP. Many of the proteins showing changed expression levels are either known targets for covalent modification by N-acetyl-p-benzoquinoneimine (NAPQI) or involved in the regulation of mechanisms that are believed to drive APAP-induced hepatotoxicity.

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.

Selective protein covalent binding and target organ toxicity

Protein covalent binding by xenobiotic metabolites has long been associated with target organ toxicity but mechanistic involvement of such binding has not been widely demonstrated. Modern biochemical, molecular, and immunochemical approaches have facilitated identification of specific protein targets of xenobiotic covalent binding. Such studies have revealed that protein covalent binding is not random, but rather selective with respect to the proteins targeted. Selective binding to specific cellular target proteins may better correlate with toxicity than total protein covalent binding. Current research is directed at characterizing and identifying the targeted proteins and clarifying the effect of such binding on their structure, function, and potential roles in target organ toxicity. The approaches employed to detect and identify the tartgeted proteins are described. Metabolites of acetaminophen, halothane, and 2,5-hexanedione form covalently bound adducts to recently identified protein targets. The selective binding may influence homeostatic or other cellular responses which in turn contribute to drug toxicity, hypersensitivity, or autoimmunity.

Selective protein arylation and acetaminophen-induced hepatotoxicity

More than 20 years have passed since the early reports of acute hepatotoxicity with APAP overdose. During that period investigative research to discover the "mechanism" underlying the toxicity has been conducted in many species and strains of intact animals as well as in a variety of in vitro and culture systems. Such work has clarified the primary role of biotransformation and the protective role of GSH. Understanding the former provides explanations for the toxic interactions which may occur with alcohol or other xenobiotics, while understanding of the latter led to the development of antidotes for the treatment of acute poisoning. Acetaminophen (APAP)-induced hepatotoxicity: roles for protein arylation. Initiating events in toxicity require biotransformation of APAP to NAPQI followed by arylation of several important proteins with subsequent alteration of protein structure and function. The immediate consequence of the alterations is detectable in several organelles and these may represent multiple initiating events which are depicted as acting in concert to cause cell injury (large arrowheads). Arylation of cytosolic 58-ABP with subsequent translocation to the nucleus is depicted as a possible signaling mechanism for determining outcome at the cell or organ level (within dotted boundary). For simplicity NAPQI's potentials for oxidizing protein sulfhydryls and direct binding to DNA have been omitted. Significant light has also been shed on the biochemical and cellular events which accompany APAP-induced hepatotoxicity. However, such studies have not identified a unique mechanism of toxicity that is universally accepted. The recent identification of several protein targets which become arylated during toxicity--along with the findings that arylation of some of those target proteins results in loss of protein function--demonstrates that covalent binding does, indeed, have biological consequences and is not merely an indicator of the fleeting presence of reactive electrophiles. These observations further suggest that multiple independent insults to the cell may be involved in toxicity. it is now apparent that the concept of a multistage process that involves both initiation and progression events is appropriate for APAP toxicity, and it is unlikely that a unique initiating event will ever be identified. In light of recent findings it is more likely that a number of such cellular events occur very early after toxic overdosage, and that they collectively set in motion and perpetuate the biochemical, cellular, and molecular processes which will determine outcome. The importance of 58-ABP arylation with early, apparently selective, translocation to the nucleus remains to be elucidated. To date there is nothing to suggest that this represents an initiating event in toxicity. rather it is plausible that the translocation may play a role in signaling electrophile presence and in calling for cellular defense against electrophile insult. This is reflected in the hypothetical model presented in Fig. 3. Critical experimental testing of this model will advance our understanding of the cellular and molecular responses to toxic electrophile insult.

Significant induction of a 54-kDa protein in rat liver with homologous alignment to mouse selenium binding protein by a coplanar polychlorinated biphenyl, 3,4,5,3',4'-pentachlorobiphenyl and 3-methylcholanthrene

A 54-kDa protein in rat liver cytosol was significantly induced by treatment with 3,4,5,3',4'-pentachlorobiphenyl (25 mg/kg, single i.p.) and 3-methylcholanthrene (20 mg/kg, once a day for 3 days, i.p.). The protein exhibited pI of 6.8 on two-dimensional gel electrophoresis. The amino acid sequences of peptide fragments from the protein digested in situ were highly similar to a selenium binding protein in mice and to the isoform acetaminophen binding protein in mice. The present result clearly demonstrates that a coplanar polychlorinated biphenyl and 3-methylcholanthrene are responsible for induction of selenium binding protein homologues. The physiological role of the mouse proteins, however, is not yet elucidated.

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.

Evidence for common binding of acetaminophen and bromobenzene to the 58-kDa acetaminophen-binding protein

Acetaminophen (APAP) toxicity has been closely associated with covalent binding to a cytosolic protein of approximately 58 kDa (58-ABP). To determine if metabolites of other toxicants might also selectively target this protein, studies were conducted with bromobenzene (BrB). Mice were given phenobarbital (80 mg/kg/d x 4 d) and were killed 4 h after challenge with 800 mg BrB/kg. Liver cytosols from BrB-treated or naive mice were incubated with an APAP activating system. Cytosolic fractions were analyzed for APAP binding by Western blotting with anti-APAP antibody. Binding to 58-ABP was selectively decreased in liver cytosol from BrB-treated mice while binding to other targets was minimally affected. Western blotting of the same samples with anti-58-ABP antisera showed that this decrease in binding did not result from diminished 58-ABP content. HPLC analysis of APAP-N-acetyl cysteine conjugate formation in vitro indicates that APAP activation was not altered in the incubates with cytosol from BrB-treated mice. These results suggest that the 58-ABP may be a common target for electrophiles in reactive intermediate toxicity.

Zonation of acetaminophen metabolism and cytochrome P450 2E1-mediated toxicity studied in isolated periportal and perivenous hepatocytes

To study the mechanism of centrilobular damage developing in the centrilobular region after high doses of acetaminophen (APAP), its metabolism and toxicity were compared in periportal and perivenous hepatocytes isolated by digitonin/collagenase perfusion. Contrary to earlier reports, based on perfusions, no evidence for a periportal dominance of APAP sulfation could be observed. Glucuronidation, the dominant pathway of conjugation at high (5 mM) APAP concentration, was faster in perivenous cells. During primary culture, prolonged exposure (> or = 24 hr) to 5 mM APAP damaged perivenous cells, with a higher P450 2E1 level than periportal cells. When cells were isolated from ethanol-pretreated rats, to induce P450 2E1 levels specifically in the perivenous region, perivenous hepatocytes exhibited enhanced APAP vulnerability and extensive glutathione depletion. In contrast, corresponding periportal cells retained good viability. Isoniazid, an inhibitor of cytochrome P450 2E1, protected cells against APAP toxicity and prevented glutathione depletion. Induction of P450 2E1 also caused a 3-fold increase in the covalent binding of reactive intermediates from [14C]APAP, and this increase was mainly confined to perivenous cells. These results indicate that in rat liver there is only slight perivenous zonation of APAP conjugation and suggest that zone-specific APAP activation, mediated by the regional expression of ethanol-inducible cytochrome P450 2E1, is responsible for the characteristic centrilobular liver damage elicited by APAP.

Purification, antibody production, and partial amino acid sequence of the 58-kDa acetaminophen-binding liver proteins

Immunochemical analysis of electrophoretically resolved liver proteins from mice administered hepatotoxic doses of acetaminophen has identified two proteins of 44 and 58 kDa as major targets for acetaminophen arylation. In the present study the 58-kDa acetaminophen-binding protein (58-ABP) was purified from mouse liver cytosol by gel permeation chromatography, preparative isoelectric focusing, and polyacrylamide gel electrophoresis. The acetaminophen adducts were visualized on immunoblots using affinity-purified anti-acetaminophen antibodies after each step of the purification. Gel permeation chromatography, under nondenaturing conditions, indicated that the protein is a monomer. Two-dimensional gel electrophoresis demonstrated that the 58-ABP consists of a cluster of four immunochemically reactive isoforms with isoelectric points ranging from 6.2 to 6.6. V-8 protease digestion of the isoforms suggested that they contained similar peptide fragments. The purified 58-ABP was utilized to produce polyclonal antibodies and to determine the amino acid composition and partial sequence of the protein. These antibodies revealed a protein cluster of similar molecular weight and isoelectric points in the cytosol of a human liver specimen. Amino acid analysis of the purified protein indicated that it contains eight cysteine residues (about 1.4% by weight). This low cysteine content raises the possibility that at hepatotoxic doses acetaminophen may also bind to non-thiol sites on the protein. The amino acid sequence of two cyanogen bromide/tryptic peptide fragments revealed that the major immunochemically detectable acetaminophen target in the cytosol is homologous to a selenium-binding protein which has been recently sequenced.

Selective alterations in the patterns of newly synthesized proteins by acetaminophen and its dimethylated analogues in primary cultures of mouse hepatocytes

Alterations in protein synthesis following exposure to and recovery from hepatotoxic doses of acetaminophen (APAP) and its analogues, 3,5-dimethyl acetaminophen (3,5-DMA) and 2,6-dimethyl acetaminophen (2,6-DMA), were investigated in primary cultures of mouse hepatocytes. The rates of protein synthesis decreased within 4 hr after administration of 10 mM APAP and occurred after significant depletion of intracellular glutathione and covalent binding of APAP to proteins, but preceded the leakage of lactate dehydrogenase into the media. The inhibition of protein synthesis was reversible only if APAP exposure did not exceed 8 hr. Electrophoretic analysis of 35S-labeled proteins by one-dimensional SDS-PAGE revealed two consistent alterations in the patterns of newly synthesized proteins. First was a progressive diminution in the de novo synthesis of a protein migrating at approximately 58 kDa (p58). This was observed with APAP (10 mM) and 3,5-DMA (5 mM) but not with 2,6-DMA (10 mM). If exposure to APAP exceeded 8 hr, the biosynthesis of this protein was not only further decreased but was also no longer detectable during the recovery period. The second major alteration was an increase in the relative rate of biosynthesis of a 32-kDa protein (p32) following exposure and recovery from APAP and 3,5-DMA but not 2,6-DMA. Exposure to heme or arsenite induced the synthesis of a protein of similar molecular weight but did not result in the inhibition of p58 biosynthesis. The fact that the reactive metabolites of both APAP and 3,5-DMA, but not 2,6-DMA, possess oxidative properties suggests that the alterations in the synthesis of p32 and p58 may be related to an oxidative component induced by these compounds.

A comparison of proteins S-thiolated by glutathione to those arylated by acetaminophen

This study was designed to evaluate whether the same proteins that irreversibly bind reactive electrophiles of drugs also bind glutathione (GSH) under oxidative conditions. Specifically, proteins that can be arylated by acetaminophen were compared to those that form glutathione-protein mixed disulfides (PSSG) after incubation with diamide. Data are presented which suggest that both GSH and acetaminophen bind to a subset of N-ethylmaleimide (NEM)-reactive protein thiols. To evaluate the pattern of proteins that bind GSH, PSSGs were formed in vitro by incubating cytosolic proteins with GSH and diamide. A sensitive procedure was developed in which PSSGs were first reduced with 0.1 mM dithiothreitol (DTT), and the newly exposed protein thiols were labeled with either [3H]NEM (for quantitative analysis) or with fluorescein-5-maleimide (for visual detection). Acetaminophen binding was achieved by incubating cytosolic proteins in vitro with the reactive acetaminophen metabolite, N-acetyl-p-benzoquinoneimine (NAPQI). Proteins from both assays were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred to nitrocellulose for Western blot analysis. Acetaminophen binding was detected by immunoblotting with an affinity-purified antibody against acetaminophen, and PSSGs were visualized using anti-fluorescein antibodies. In both instances, binding to proteins was observed to be selective. A comparison of the proteins modified by GSH binding with those that bind acetaminophen indicates that the major cytosolic acetaminophen-binding protein of 58 kDa may also be modified by glutathiolation under oxidative conditions.

Comparative cytotoxic effects of acetaminophen (N-acetyl-p-aminophenol), a non-hepatotoxic regioisomer acetyl-m-aminophenol and their postulated reactive hydroquinone and quinone metabolites in monolayer cultures of mouse hepatocytes

Toxic effects of acetaminophen (paracetamol, N-acetyl-p-aminophenol, APAP) in monolayer cultures of mouse hepatocytes developed over a period of 18 hr. N-Acetyl-m-aminophenol (AMAP) was approximately 10-fold less toxic than APAP, despite the fact that it bound covalently to a greater extent to hepatocyte macromolecules. AMAP did not deplete glutathione to as great an extent as APAP, indicating that their reactive metabolites may bind to different proteins or that oxidative damage in addition to arylation of proteins may be involved in the development of cell death. The toxicity of 3-methoxy-acetyl-p-aminophenol was similar to that of APAP, whereas the other hydroquinone and quinone metabolites were 8-10 times more cytotoxic than APAP. The potencies of these analogs were in the order: acetyl-m-aminophenol-p-benzoquinoneimine greater than or equal to 2,5-dihydroxyacetanilide greater than or equal to 3-methoxy-p-benzoquinone greater than or equal to N-acetyl-p-benzoquinone imine (NAPQI) greater than or equal to acetyl-m-aminophenol-o-benzoquinone greater than or equal to 3-hydroxy-acetyl-p-aminophenol. The relative toxic potencies of the hydroquinone and quinone metabolites of AMAP were comparable to that of NAPQI, and do not readily explain the marked difference between the cytotoxic effects of AMAP and APAP.

Acetaminophen hepatotoxicity: correspondence of selective protein arylation in human and mouse liver in vitro, in culture, and in vivo

Human and mouse liver were exposed to an APAP-activating system, in vitro. Subsequent immunochemical analysis of electrophoretically separated proteins with an affinity-purified anti-APAP antibody indicated that when a cytosolic fraction from human liver was incubated with APAP, an NADPH-regenerating system, and mouse microsomes selective APAP binding occurred predominantly to proteins of approximately 38, 58, and 130 kDa. To evaluate whether similar proteins are targeted in situ, primary cultures of human hepatocytes were treated with 10 mM APAP for 4 hr prior to immunochemical analysis. APAP binding was again detected in protein bands of approximately 38, 58, and 130 kDa. In addition, selective binding was also noted to other cytosolic protein bands, e.g., approximately 52 and 62 kDa. For mouse liver, the majority of the binding, in vitro or in culture, was to proteins of approximately 44 and 58 kDa with lesser binding to proteins of approximately 33 and 130 kDa among others. By contrast, at the times monitored, little covalent binding was detected in the 44-kDa region in the human liver experiments. Most noteworthy was the finding that when the protein arylation patterns on liver samples from a human APAP fatality were compared to those from a mouse given a hepatotoxic dose of APAP, the binding patterns were similar to those detected after the in vitro and the culture experiments with mouse and human livers. Furthermore, an immunohistochemical analysis revealed that as with the mouse, APAP covalent binding in the human liver exhibited a distinct zonal pattern consistent with centrilobular binding. That APAP arylation of the 58- and 130-kDa proteins was observed in livers from both mice and humans suggests that the mouse provides a valid model for studying the mechanistic importance of covalent binding. Elucidation of the identities and functions of the common targeted proteins may clarify their toxicological significance.

Acetaminophen structure-toxicity studies: in vivo covalent binding of a nonhepatotoxic analog, 3-hydroxyacetanilide

High doses of 3-hydroxyacetanilide (3HAA), a structural isomer of acetaminophen, do not produce hepatocellular necrosis in normal male hamsters or in those sensitized to acetaminophen-induced liver damage by pretreatment with a combination of 3-methylcholanthrene, borneol, and diethyl maleate. Although 3HAA was not hepatotoxic, the administration of acetyl-labeled [3H or 14C]3HAA (400 mg/kg, ip) produced levels of covalently bound radiolabel that were similar to those observed after an equimolar, hepatotoxic dose of [G-3H]acetaminophen. The covalent nature of 3HAA binding was demonstrated by retention of the binding after repetitive organic solvent extraction following protease digestion. Hepatic and renal covalent binding after 3HAA was approximately linear with both dose and time. In addition, 3HAA produced only a modest depletion of hepatic glutathione, suggesting the lack of a glutathione threshold. 3-Methylcholanthrene pretreatment increased and pretreatment with cobalt chloride and piperonyl butoxide decreased the hepatic covalent binding of 3HAA, indicating the involvement of cytochrome P450 in the formation of the 3HAA reactive metabolite. The administration of multiple doses or a single dose of [ring-3H]3HAA to hamsters pretreated with a combination of 3-methylcholanthrene, borneol, and diethyl maleate produced hepatic levels of 3HAA covalent binding that were in excess of those observed after a single, hepatotoxic acetaminophen dose. These data suggest that the nature and/or the intracellular processing of the reactive metabolites of acetaminophen and 3HAA are different. These data also demonstrate that absolute levels of covalently bound xenobiotic metabolites cannot be utilized as absolute predictors of cytotoxic potential.