Covalent modification of lipids and proteins in rat hepatocytes and in vitro by thioacetamide metabolites

Absence of Increased Susceptibility to Acetaminophen-Induced Liver Injury in a Diet-Induced NAFLD Mouse Model

Nonalcoholic fatty liver disease (NAFLD) is a common chronic liver disease and its influence on drug-induced liver injury (DILI) is not fully understood. We investigated whether NAFLD can influence acetaminophen (APAP [N-acetyl-p-aminophenol])-induced hepatotoxicity in a diet-induced obese (DIO) mouse model of NAFLD. The male C57BL/6NTac DIO mice, fed a high-fat diet for more than 12 weeks, developed obesity, hyperinsulinemia, impaired glucose tolerance, and hepatomegaly with hepatic steatosis, similar to human NAFLD. In the acute toxicity study after a single dose of APAP (150 mg/kg), compared with control lean mice, the DIO mice had decreased serum transaminase levels and less severe hepatocellular injury. The DIO mice also had altered expression of genes related to APAP metabolism. Chronic APAP exposure for 26 weeks did not predispose the DIO mice with NAFLD to more severe hepatotoxicity compared with the lean mice. These results suggested that the C57BL/6NTac DIO mouse model appears to be more tolerant to APAP-induced hepatotoxicity than lean mice, potentially related to altered xenobiotic metabolizing capacity in the fatty liver. Further mechanistic studies with APAP and other drugs in NAFLD animal models are necessary to investigate the mechanism of altered susceptibility to intrinsic DILI in some human NAFLD patients.

Asparagine and Glutamine Residues Participate in Protein Covalent Binding by Epoxide Metabolite of 8-Epidiosbulbin E Acetate In Vitro and In Vivo

Dioscorea Bulbifera L. (DBL), an effective traditional Chinese medicine, has been restricted because of multiple reports that it can cause severe hepatotoxicity. 8-Epidiosbulbin E acetate (EEA), one of the main components of DBL, can induce severe liver injury. It has been reported that EEA can be metabolized by CYP3A to the corresponding cis-enedial intermediate which alkylates the lysine residues of proteins to form pyrroline derivatives. The present study unexpectedly found that the reactive intermediate reacted with the amide groups of asparagine (Asn) and glutamine (Gln) residues of hepatic proteins of mice treated with EEA. The amide-derived protein modification increased with the increase in the dose administered. Like the adduction of the primary amine of lysine residues, the electrophilic metabolite reacted with the amide groups of Asn and Gln residues to offer the corresponding pyrrolines. The structures of the pyrrolines were confirmed by mass spectrometry and nuclear magnetic resonance spectroscopy.

Further Insights into the Oxidative Pathway of Thiocarbonyl-Type Antitubercular Prodrugs: Ethionamide, Thioacetazone, and Isoxyl

A chemical activation study of the thiocarbonyl-type antitubercular prodrugs, ethionamide (ETH), thioacetazone (TAZ), and isoxyl (ISO), was performed. Biomimetic oxidation of ethionamide using H₂O₂ (1 equiv) led to ETH-SO as the only stable S-oxide compound, which was found to occur in solution in the preferential form of a sulfine (ETH═S═O vs the sulfenic acid tautomer ETH-S-OH), as previously observed in the crystal state. It was also demonstrated that ETH-SO is capable of reacting with amines, as the putative sulfinic derivative (ETH-SO₂H) was supposed to do. Unlike ETH, oxidation of TAZ did not allow observation of the mono-oxygenated species (TAZ-SO), leading directly to the more stable sulfinic acid derivative (TAZ-SO₂H), which can then lose a SO_xH group after further oxidation or when placed in a basic medium. It was also noticed that the unstable TAZ-SO intermediate can lead to the carbodiimide derivative as another electrophilic species. It is suggested that TAZ-SOH, TAZ-SO₂H, and the carbodiimide compound can also react with NH₂-containing nucleophilic species, and therefore be involved in toxic effects. Finally, ISO showed a very complex reactivity, here assigned to the coexistence of two mono-oxygenated structures, the sulfine and sulfenic acid tautomers. The mono- and dioxygenated derivatives of ISO are also highly unstable, leading to a panel of multiple metabolites, which are still reactive and likely contribute to the toxicity of this prodrug.

Effects of dexamethasone on hepatic macrophages in normal livers and thioacetamide-induced acute liver lesions in rats

Resident and infiltrative macrophages play important roles in the development of pathological lesions. M1/M2 macrophage polarization with respective CD68 and CD163 expression remains unclear in chemically induced liver injury. This study was aimed at investigating the influence of macrophages on normal and chemically induced liver injury. For this, dexamethasone (DX), an immunosuppressive drug, was administered in normal rats and thioacetamide (TAA)-treated rats. Liver samples were collected and analyzed with immunohistochemical methods. Repeated injections of DX (0.5 or 1.0 mg/kg BW) for 3, 7 and 11 days reduced the number of CD163 positive hepatic resident macrophages (Kupffer cells) in normal livers, while increasing AST and ALT levels. In TAA (300 mg/kg BW)-treated rats injected with DX (0.5 mg/kg BW) pretreatment, the number of M1 and M2 macrophages showed a significant decrease compared with that of TAA-treated rats without DX treatment. Additionally, reparative fibrosis resulting from hepatocyte injury induced by TAA injection was suppressed by DX pretreatment. Our data suggested that macrophages could influence not only normal hepatic homeostasis (reflected by AST and ALT levels) but also chemically induced hepatic lesion development (reduced reparative fibrosis).

Possible Pathways of Hepatotoxicity Caused by Chemical Agents

BACKGROUND

Liver injury induced by drugs has become a primary reason for acute liver disease and therefore posed a potential regulatory and clinical challenge over the past few decades and has gained much attention. It also remains the most common cause of failure of drugs during clinical trials. In 50% of all acute liver failure cases, drug-induced hepatoxicity is the primary factor and 5% of all hospital admissions.

METHODS

The various hepatotoxins used to induce hepatotoxicity in experimental animals include paracetamol, CCl4, isoniazid, thioacetamide, erythromycin, diclofenac, alcohol, etc. Among the various models used to induce hepatotoxicity in rats, every hepatotoxin causes toxicity by different mechanisms.

RESULTS

The drug-induced hepatotoxicity caused by paracetamol accounts for 39% of the cases and 13% hepatotoxicity is triggered by other hepatotoxic inducing agents.

CONCLUSION

Research carried out and the published papers revealed that hepatotoxins such as paracetamol and carbon- tetrachloride are widely used for experimental induction of hepatotoxicity in rats.

Antimicrobial Peptide-Polymer Conjugates for Dentistry

Bacterial adhesion and growth at the composite/adhesive/tooth interface remain the primary cause of dental composite restoration failure. Early colonizers, including Streptococcus mutans, play a critical role in the formation of dental caries by creating an environment that reduces the adhesive's integrity. Subsequently, other bacterial species, biofilm formation, and lactic acid from S. mutans demineralize the adjoining tooth. Because of their broad spectrum of antibacterial activity and low risk for antibiotic resistance, antimicrobial peptides (AMPs) have received significant attention to prevent bacterial biofilms. Harnessing the potential of AMPs is still very limited in dentistry-a few studies have explored peptide-enabled antimicrobial adhesive copolymer systems using mainly nonspecific adsorption. In the current investigation, to avoid limitations from nonspecific adsorption and to prevent potential peptide leakage out of the resin, we conjugated an AMP with a commonly used monomer for dental adhesive formulation. To tailor the flexibility between the peptide and the resin material, we designed two different spacer domains. The spacer-integrated antimicrobial peptides were conjugated to methacrylate (MA), and the resulting MA-AMP monomers were next copolymerized into dental adhesives as AMP-polymer conjugates. The resulting bioactivity of the polymethacrylate-based AMP conjugated matrix activity was investigated. The antimicrobial peptide conjugated to the resin matrix demonstrated significant antimicrobial activity against S. mutans. Secondary structure analyses of conjugated peptides were applied to understand the activity differential. When mechanical properties of the adhesive system were investigated with respect to AMP and cross-linking concentration, resulting AMP-polymer conjugates maintained higher compressive moduli compared to hydrogel analogues including polyHEMA. Overall, our result provides a robust approach to develop a fine-tuned bioenabled peptide adhesive system with improved mechanical properties and antimicrobial activity. The results of this study represent a critical step toward the development of peptide-conjugated dentin adhesives for treatment of secondary caries and the enhanced durability of dental composite restorations.

Mice deficient in aldo-keto reductase 1a (Akr1a) are resistant to thioacetamide-induced liver injury

Aldehyde reductase (Akr1a) has been reported to be involved in detoxification of reactive aldehydes as well as in the synthesis of bioactive compounds such as ascorbic acid (AsA). Because Akr1a is expressed at high levels in the liver and is involved in xenobiotic metabolism, our objective was to investigate the hepato-protective role of Akr1a in a thioacetamide (TAA)-induced hepatotoxicity model using Akr1a-deficient (Akr1a^-/-) mice. Wild-type (WT) and Akr1a^-/- mice were injected intraperitoneally with TAA and the extent of liver injury in the acute phase was assessed. Intriguingly, the extent of TAA-induced liver damage was less in the Akr1a^-/- mice than in the WT mice. Biomarkers for the ER stress-induced apoptosis pathway were markedly decreased in the livers of Akr1a^-/- mice, whereas AsA levels in plasma did not change significantly in any of the mice. In the liver, TAA is converted to reactive metabolites such as TAA S-oxide and then to TAA S, S-dioxide via the action of CYP2E1. In Akr1a^-/- mice, CYP2E1 activity was relatively lower than WT mice at the basal level, leading to reactive TAA metabolites being produced at lower levels after the TAA treatment. The levels of liver proteins that were modified with these metabolites were also lower in the Akr1a^-/- mice than the WT mice after the TAA treatment. Furthermore, after a lethal dose of a TAA challenge, the WT mice all died within 36 h, whereas almost all of the Akr1a^-/- mice survived. These collective results suggest that Akr1a^-/- mice are resistant to TAA-induced liver injury, and it follows that the absence of Akr1a might modulate TAA bioactivation.

Role of Oxygen Free Radicals, Nitric Oxide and Mitochondria in Mediating Cardiac Alterations During Liver Cirrhosis Induced by Thioacetamide

Thioacetamide (TAA) administration is widely used for induction of liver cirrhosis in rats, where reactive oxygen radicals (ROS) and nitric oxide (NO) participate in development of liver damage. Cardiac dysfunction is an important complication of liver cirrhosis, but the role of ROS or NO in cardiac abnormalities during liver cirrhosis is not well understood. This was investigated in animals after TAA-induced liver cirrhosis and temporal changes in oxidative stress, NO and mitochondrial function in the heart evaluated. TAA induced elevation in cardiac levels of nitrate before development of frank liver cirrhosis, without gross histological alterations. This was accompanied by an early induction of P38 MAP kinase, which is influenced by ROS and plays an important signaling role for induction of iNOS. Increased nitrotyrosine, protein oxidation and lipid peroxidation in the heart and cardiac mitochondria, suggestive of oxidative stress, also preceded frank liver cirrhosis. However, compromised cardiac mitochondrial function with a decrease in respiratory control ratio and increased mitochondrial swelling was seen later, when cirrhosis was evident. In conclusion, TAA induces elevations in ROS and NO in the heart in parallel to early liver damage. This leads to later development of functional deficits in cardiac mitochondria after development of liver cirrhosis.

Attenuation of thioacetamide-induced hepatocellular injury by short-term repeated injections associated with down-regulation of metabolic enzymes and relationship with MHC class II-presenting cells

The liver is the primary organ participating in the metabolism of xenobiotics and is therefore an important target in the safety assessment of drugs, chemicals and environmental toxins. Drug-induced liver injury (DILI) has recently become widely recognized in human medicine as an adverse event. The progression of DILI often involves "damage-associated molecular patterns" (DAMPs) of gene and protein expression such as high-mobility group boxes (HMGBs), S100 proteins and heat shock proteins (Hsp). DAMPs are released from injured or necrotic cells and are bound to Toll-like receptors (TLRs) and modulate inflammatory reactions. Previously, in thioacetamide (TAA; 300mg/kg body weight, single injection)-induced rat liver, we demonstrated that the expressions of DAMPs, TLR4 and major histocompatibility complex (MHC) class II were simultaneously increased, accompanied with progression of hepatocellular injury and inflammation. Here we investigated the association of DILI and DAMPs, TLRs and MHC class II by using rat livers repeated injections with TAA (100mg/kg body weight, once, three times). Two days after TAA single injection, centrilobular hepatocellular necrosis with infiltration of mononuclear cells was observed, being paralleled with increase in serum levels of aspartate transaminase (AST), alanine transaminase (ALT) and alkaline phosphatase (ALP). However, two days after duplicate and triplicate injections, only mild degenerative change of hepatocytes and slight infiltration of mononuclear cells were seen in the affected centrilobular area. Serum levels of AST, ALT and ALP were also decreased to the same levels of control. mRNA expressions of DAMPs (HMGBs, S100A4 and Hsp 70-2), TLR4 and MHC class II tended to be increased only on single injection, although the number of MHC class II-positive cells in the centrilobular area was still increased on each examination point. The analysis of enzymes (CYP2E1 and Flavin monooxygenase (FMO) 3), which metabolize TAA in hepatocytes, showed a significant decrease in FMO3 on the duplicate and triplicate injections. Autophagy and regulatory T cells were not significantly changed for the attenuation of hepatocyte injury. Collectively, these results suggest that hepatocytes may adapt accumulation of the toxicant by changing their enzyme functions; furthermore, MHC class II cells, which still showed increased number in the duplicate and triplicate injections, may be related with protection from the toxicant.

Glutathione Adduct Patterns of Michael-Acceptor Carbonyls

Glutathione (GSH) has so far been considered to facilitate detoxification of soft organic electrophiles through covalent binding at its cysteine (Cys) thiol group, followed by stepwise catalyzed degradation and eventual elimination along the mercapturic acid pathway. Here we show that in contrast to expectation from HSAB theory, Michael-acceptor ketones, aldehydes and esters may form also single, double and triple adducts with GSH involving β-carbon attack at the much harder N-terminus of the γ-glutamyl (Glu) unit of GSH. In particular, formation of the GSH-N single adduct contradicts the traditional view that S alkylation always forms the initial reaction of GSH with Michael-acceptor carbonyls. To this end, chemoassay analyses of the adduct formation of GSH with nine α,β-unsaturated carbonyls employing high performance liquid chromatography and tandem mass spectrometry have been performed. Besides enriching the GSH adductome and potential biomarker applications, electrophilic N-terminus functionalization is likely to impair GSH homeostasis substantially through blocking the γ-glutamyl transferase catalysis of the first breakdown step of modified GSH, and thus its timely reconstitution. The discussion includes a comparison with cyclic adducts of GSH and furan metabolites as reported in literature, and quantum chemically calculated thermodynamics of hard-hard, hard-soft, and soft-soft adducts.

Protection against Acute Hepatocellular Injury Afforded by Liver Fibrosis Is Independent of T Lymphocytes

Collagen produced during the process of liver fibrosis can induce a hepatocellular protective response through ERK1 signalling. However, the influence of T cells and associated cytokine production on this protection is unknown. In addition, athymic mice are frequently used in hepatocellular carcinoma xenograft experiments but current methods limit our ability to study the impact of liver fibrosis in this setting due to high mortality. Therefore, a mouse model of liver fibrosis lacking T cells was developed using Foxn1 nu/nu mice and progressive oral administration of thioacetamide (TAA) [0.01–0.02%] in drinking water. Fibrosis developed over a period of 16 weeks (alpha-SMA positive area: 20.0 ± 2.2%, preCol1a1 mRNA expression: 11.7 ± 4.1 fold changes, hydroxyproline content: 1041.2 ± 77μg/g of liver) at levels comparable to that of BALB/c mice that received intraperitoneal TAA injections [200 μg/g of body weight (bw)] (alpha-SMA positive area: 20.9 ± 2.9%, preCol1a1 mRNA expression: 13.1 ± 2.3 fold changes, hydroxyproline content: 931.6 ± 14.8μg/g of liver). No mortality was observed. Athymic mice showed phosphorylation of ERK1/2 during fibrogenesis (control 0.03 ± 0.01 vs 16 weeks 0.22 ± 0.06AU; P<0.05). The fibrosis-induced hepatoprotection against cytotoxic agents, as assessed histologically and by serum AST levels, was not affected by the absence of circulating T cells (anti-Fas JO2 [0.5μg/g bw] for 6h (fibrotic 4665 ± 2596 vs non-fibrotic 13953 ± 2260 U/L; P<0.05), APAP [750 mg/kg bw] for 6 hours (fibrotic 292 ± 66 U/L vs non-fibrotic 4086 ± 2205; P<0.01) and CCl₄ [0.5mL/Kg bw] for 24h (fibrotic 888 ± 268 vs non-fibrotic 15673 ± 2782 U/L; P<0.001)). In conclusion, liver fibrosis can be induced in athymic Foxn1 nu/nu mice without early mortality. Liver fibrosis leads to ERK1/2 phosphorylation. Finally, circulating T lymphocytes and associated cytokines are not involved in the hepatocellular protection afforded by liver fibrosis.

Histopathological Analysis of Rat Hepatotoxicity Based on Macrophage Functions: in Particular, an Analysis for Thioacetamide-induced Hepatic Lesions

Hepatic macrophages play an important role in homeostasis. The functional abnormalities of hepatic macrophages primarily or secondarily influence chemically induced hepatotoxicity. However, the evaluation system based on their functions has not yet been established. Recently, a new concept (M1-/M2-macrophage polarization) was proposed; M1-macropahges are induced by INF-γ, and show high phagocytosis/tissue damage, whereas M2-macropahges are induced by IL-4 and play roles in reparative fibrosis by releasing IL-10 and TGF-β1. In hepatogenesis, CD68-expressing M1-macrophages predominantly exist in embryos; in neonates, in contrast, CD163-/CD204-expressing M2-macrophages appear along the sinusoids and mature as Kupffer cells. Activated Kupffer cells by liposome decrease AST and ALT values, whereas AST and ALT values are increased under Kupffer cells depleted with clodronate treatment. Since Kupffer cells may be involved in clearance of liver enzymes, macrophage condition should be taken into consideration when hepatotoxicity is analyzed. In TAA-induced acute hepatic lesions, INF-γ, TNF-α and IL-6 for M1-factors and IL-4 for M2-factors are already increased before histopathological change; the appearance of CD68-expressing M1-macrophages and CD163-expressing M2-macrophages follows in injured centrilobular lesions, and TGF-β1 and IL-10 are increased for reparative fibrosis. CD68-expressing M1-macrophages co-express MHC class II and Iba-1, whereas CD163-expressing M2-macrophages also express CD204 and Galectin-3. Under macrophage depletion by clodoronate, TAA-treated rat livers show prolonged coagulation necrosis of hepatocytes, and then develop dystrophic calcification without reparative fibrosis. The depletion of hepatic macrophages influences hepatic lesion development. Collectively, a histopathological analysis method for hepatotoxicity according to M1-/M2-macrophage polarization would lead to the refinement of hazard characterization of chemicals in food and feed.

The kinetics of damage-associated molecular patterns (DAMPs) and toll-like receptors during thioacetamide-induced acute liver injury in rats

Drug-induced liver injury (DILI) is a common problem in human medicine and it is a major reason to withdraw marketed drugs. However, the mechanism of DILI is still less known. Damage-associated molecular patterns (DAMPs), such as high-mobility group boxes (HMGBs), S100 proteins and heat shock proteins (HSPs), are released from injured or necrotic cells, bind to toll-like receptors (TLRs) and modulate inflammatory reactions. Here we investigated the kinetics of DAMPs, TLRs and MHC class II in a rat model of DILI with thioacetamide (TAA). After TAA administration, extensive necrosis was observed on days 1 and 2, followed by infiltration of inflammatory cells on day 3. The levels of serum liver enzymes also peaked on day 1. Expression of HMGB-1, -2 and S100A4 peaked on day 2. TLR-4 was up-regulated on day 3. The number of MHC class II-positive macrophages increased until day 2. These results suggest that HMGB-1, -2 and S100A4 are associated with hepatocellular necrosis and that DAMPs may activate TLR-4 and MHC class II during TAA-induced liver injury. Our data would contribute to the elucidation of the mechanism of DILI.

Oxidative stress caused by a SOD1 deficiency ameliorates thioacetamide-triggered cell death via CYP2E1 inhibition but stimulates liver steatosis

We investigated the responses of mice that are defective in the superoxide-scavenging enzyme SOD1 to thioacetamide (TAA)-induced hepatotoxicity. When a lethal dose of TAA (500 mg/kg) was intraperitoneally injected, the wild-type (WT) mice all died within 36 h, but all of the SOD1-knockout (KO) mice survived. Treatment with an SOD1 inhibitor rendered the WT mice resistant to TAA toxicity. To elucidate the mechanism responsible for this, we examined the acute effects of a sublethal dose of TAA (200 mg/kg) on the livers of WT and KO mice. The extent of TAA-induced liver damage was less in the KO mice, but, instead, lipogenesis was further advanced in the SOD1-KO livers. The levels of proteins modified with acetyllysine, a marker for TAA-mediated injury, were lower in the KO mice than the WT mice upon the TAA treatment. The KO mice, which were under oxidative stress per se, exhibited a lower CYP2E1 activity, and this appeared to result in a decrease in the production of reactive oxygen species (ROS) during TAA metabolism. Both cleaved ATF6, a transcriptional regulator that is activated by endoplasmic reticulum (ER) stress, and CHOP, a death signal mediator, were highly elevated in the WT mice as the result of the TAA treatment and consistent with the liver damage. We conclude that elevated TAA metabolites and reactive oxygen species that are produced by CYP-mediated drug metabolism trigger lipogenesis as well as liver damage via ER stress and determine the fate of the mice.

Standard operating procedures in experimental liver research: thioacetamide model in mice and rats

In addition to carbon tetrachloride (CCl4), thioacetamide (TAA) represents a second widely used model for the induction of experimental liver fibrosis, but can also be employed for the development of acute liver failure and liver tumours. While TAA itself is not hepatotoxic, its reactive metabolites covalently bind to proteins and lipids thereby causing oxidative stress and centrilobular necrosis. Compared with CCl4, TAA leads to more periportal infiltrates and more pronounced ductal proliferation. While TAA has been shown to induce liver fibrosis development in several different mouse strains, wide variations in the administration routes, doses and treatment durations have been reported. Therefore, an adoption of a universal standard operating procedure for the administration of TAA is urgently needed. For that purpose, we are presenting here two TAA models (intraperitoneal administration of 150 mg/kg of TAA three times per week for 11 weeks in rats, and TAA administration in drinking water at 300 mg/L for 2-4 months in mice) with which we have had success in reliably and reproducibly developing chronic liver injury and fibrosis.

Increased serum bile acid concentration following low-dose chronic administration of thioacetamide in rats, as evidenced by metabolomic analysis

A liquid chromatography/time-of-flight mass spectrometry (LC/TOF-MS)-based metabolomics approach was employed to identify endogenous metabolites as potential biomarkers for thioacetamide (TAA)-induced liver injury. TAA (10 and 30mg/kg), a well-known hepatotoxic agent, was administered daily to male Sprague-Dawley (SD) rats for 28days. We then conducted untargeted analyses of endogenous serum and liver metabolites. Partial least squares discriminant analysis (PLS-DA) was performed on serum and liver samples to evaluate metabolites associated with TAA-induced perturbation. TAA administration resulted in altered levels of bile acids, acyl carnitines, and phospholipids in serum and in the liver. We subsequently demonstrated and confirmed the occurrence of compromised bile acid homeostasis. TAA treatment significantly increased serum levels of conjugated bile acids in a dose-dependent manner, which correlated well with toxicity. However, hepatic levels of these metabolites were not substantially changed. Gene expression profiling showed that the hepatic mRNA levels of Ntcp, Bsep, and Oatp1b2 were significantly suppressed, whereas those of basolateral Mrp3 and Mrp4 were increased. Decreased levels of Ntcp, Oatp1b2, and Ostα proteins in the liver were confirmed by western blot analysis. These results suggest that serum bile acids might be increased due to the inhibition of bile acid enterohepatic circulation rather than increased endogenous bile acid synthesis. Moreover, serum bile acids are a good indicator of TAA-induced hepatotoxicity.

Protein targets of thioacetamide metabolites in rat hepatocytes

Thioacetamide (TA) has long been known as a hepatotoxicant whose bioactivation requires S-oxidation to thioacetamide S-oxide (TASO) and then to the very reactive S,S-dioxide (TASO2). The latter can tautomerize to form acylating species capable of covalently modifying cellular nucleophiles including phosphatidylethanolamine (PE) lipids and protein lysine side chains. Isolated hepatocytes efficiently oxidize TA to TASO but experience little covalent binding or cytotoxicity because TA is a very potent inhibitor of the oxidation of TASO to TASO2. However, hepatocytes treated with TASO show extensive covalent binding to both lipids and proteins accompanied by extensive cytotoxicity. In this work, we treated rat hepatocytes with [(14)C]-TASO and submitted the mitochondrial, microsomal, and cytosolic fractions to 2DGE, which revealed a total of 321 radioactive protein spots. To facilitate the identification of target proteins and adducted peptides, we also treated cells with a mixture of TASO/[(13)C2D3]-TASO. Using a combination of 1DGE- and 2DGE-based proteomic approaches, we identified 187 modified peptides (174 acetylated, 50 acetimidoylated, and 37 in both forms) from a total of 88 nonredundant target proteins. Among the latter, 57 are also known targets of at least one other hepatotoxin. The formation of both amide- and amidine-type adducts to protein lysine side chains is in contrast to the exclusive formation of amidine-type adducts with PE phospholipids. Thiobenzamide (TB) undergoes the same two-step oxidative bioactivation as TA, and it also gives rise to both amide and amidine adducts on protein lysine side chains but only amidine adducts to PE lipids. Despite their similarity in functional group chemical reactivity, only 38 of 62 known TB target proteins are found among the 88 known targets of TASO. The potential roles of protein modification by TASO in triggering cytotoxicity are discussed in terms of enzyme inhibition, protein folding, and chaperone function, and the emerging role of protein acetylation in intracellular signaling and the regulation of biochemical pathways.

Metabolism and toxicity of thioacetamide and thioacetamide S-oxide in rat hepatocytes

The hepatotoxicity of thioacetamide (TA) has been known since 1948. In rats, single doses cause centrolobular necrosis accompanied by increases in plasma transaminases and bilirubin. To elicit these effects, TA requires oxidative bioactivation, leading first to its S-oxide (TASO) and then to its chemically reactive S,S-dioxide (TASO(2)), which ultimately modifies amine-lipids and proteins. To generate a suite of liver proteins adducted by TA metabolites for proteomic analysis and to reduce the need for both animals and labeled compounds, we treated isolated hepatocytes directly with TA. Surprisingly, TA was not toxic at concentrations up to 50 mM for 40 h. On the other hand, TASO was highly toxic to isolated hepatocytes as indicated by LDH release, cellular morphology, and vital staining with Hoechst 33342/propidium iodide. TASO toxicity was partially blocked by the CYP2E1 inhibitors diallyl sulfide and 4-methylpyrazole and was strongly inhibited by TA. Significantly, we found that hepatocytes produce TA from TASO relatively efficiently by back-reduction. The covalent binding of [(14)C]-TASO is inhibited by unlabeled TA, which acts as a "cold-trap" for [(14)C]-TA and prevents its reoxidation to [(14)C]-TASO. This in turn increases the net consumption of [(14)C]-TASO despite the fact that its oxidation to TASO(2) is inhibited. The potent inhibition of TASO oxidation by TA, coupled with the back-reduction of TASO and its futile redox cycling with TA, may help explain phenomena previously interpreted as "saturation toxicokinetics" in the in vivo metabolism and toxicity of TA and TASO. The improved understanding of the metabolism and covalent binding of TA and TASO facilitates the use of hepatocytes to prepare protein adducts for target protein identification.