Role of CYP2E1 and saturation kinetics in the bioactivation of thioacetamide: Effects of diet restriction and phenobarbital

Molecular mechanisms in thioacetamide-induced acute and chronic liver injury models

Thioacetamide (TAA) undergoes bioactivation in the liver by the CYP450 2E1 enzyme, resulting in the formation of TAA-S-oxide and TAA-S-dioxide. TAA-S-dioxide induces oxidative stress via lipid peroxidation of the hepatocellular membrane. A single TAA dose (50-300 mg/kg) administration initiates hepatocellular necrosis around the pericentral region after its covalent binding to macromolecules in the liver. Intermittent TAA administration (150-300 mg/kg, weekly thrice, for 11-16 weeks) activates transforming growth factor (TGF)-β/smad3 downstream signaling in injured hepatocytes, causing hepatic stellate cells (HSCs) to acquire myofibroblast like phenotype. The activated HSCs synthesize a variety of extracellular matrix, leading to liver fibrosis, cirrhosis, and portal hypertension. The TAA induced liver injury varies depending on the animal model, dosage, frequency, and routes of administration. However, TAA induces hepatotoxicity in a reproducible manner, and it is an ideal model to evaluate the antioxidant, cytoprotective, and antifibrotic compounds in experimental animals.

Lycorine Ameliorates Thioacetamide-Induced Hepatic Fibrosis in Rats: Emphasis on Antioxidant, Anti-Inflammatory, and STAT3 Inhibition Effects

Liver fibrosis is a foremost medical concern worldwide. In Saudi Arabia, numerous risk factors contribute to its high rates. Lycorine—a natural alkaloid—has antioxidant, anti-inflammatory, and antitumor activates. It has been reported to inhibit STAT3 in cancer. Therefore, this study aimed at investigating the possible antifibrotic effect of lycorine against thioacetamide (TAA)-induced liver fibrosis in rats and at elucidating the possible mechanisms. Liver fibrosis was induced by TAA (200 mg/kg i.p.), three per week for four weeks. Treatment with lycorine (0.5 and 1 mg/kg/d) amended TAA-induced rise of serum transaminases that was confirmed histopathologically. Moreover, it ameliorated liver fibrosis in a dose-dependent manner, as indicated by hindering the TAA-induced increase of hepatic hydroxyproline content, α-smooth muscle actin (α-SMA) and transforming growth factor (TGF-β1) expressions. TAA-induced oxidative stress was amended by lycorine treatment via restoring reduced glutathione and diminishing lipid peroxidation. Moreover, lycorine ameliorated hepatic inflammation by preventing the rise of inflammatory cytokines. Notably, lycorine inhibited STAT3 activity, as evidenced by the decreased phospho-STAT3 expression, accompanied by the elevation of the hepatic Bax/Bcl-2 ratio. In conclusion, lycorine hinders TAA-induced liver fibrosis in rats, due to—at least partly—its antioxidative and anti-inflammatory properties, along with its ability to inhibit STAT3 signaling.

P2X7R orchestrates the progression of murine hepatic fibrosis by making a feedback loop from macrophage to hepatic stellate cells

HSCs (hepatic stellate cells) contribute to the excessive extracellular matrix (ECM) deposition, inflammatory pathways and crucial cell-cell interactions in hepatic disease leading to fibrosis. P2x7R is considered a potential orchestrater in liver fibrosis. For this reason, this work explored the role of P2x7R in liver fibrosis and the mechanism by which P2x7R in macrophages promotes fibrogenesis. In a model of liver fibrosis induced by administration of thioacetamide (TAA), inhibition of P2x7R with its selective inhibitor A438079 reversed TAA-induced liver damage and fibrosis. The mechanism was linked to inhibition of P2x7R-NLRP3 inflammasome activation and thereby infiltration of macrophages and neutrophils into the liver. This result indicated that the P2x7R-TLR4-NLRP3 axis is involved in the process of TGF-β-mediated ECM deposition in HSCs. Ectopic overexpression of P2x7R lowered the threshold of extracellular matrix (ECM) deposition and maintained HSCs in an activated state. The culture medium of THP-1 macrophages stimulated by LPS/ATP aggravated ECM deposition in HSCs by activating P2x7R. Additionally, IL-1β secreted by LPS / ATP activated macrophages amplified fibrosis. These data indicate that P2x7R plays a key regulative role in the activation and maintenance of HSCs promoted by macrophages. Thus, pharmacological inhibition of P2x7R could be a potential therapeutic mechanism to treat human liver fibrosis.

Mice in the early stage of liver steatosis caused by a high fat diet are resistant to thioacetamide-induced hepatotoxicity and oxidative stress

Lipogenesis is stimulated in the liver by an unfolded protein response (UPR) to endoplasmic reticulum stress under a variety of pathological conditions and results in the accumulation of lipids in hepatocytes. Assuming that UPR is a protective mechanism against stress, we hypothesized that the accumulated lipids might have a beneficial function. We prepared mice with fatty livers by feeding two types of high-calorie diets; a lard-rich high-calorie diet (LHD) or a menhaden oil-containing high-calorie diet (MHD), for two weeks and treated them, as well as control diet (CD)-fed mice, with thioacetamide (TAA), a liver toxicant. When a lethal dose (500mg/kg) of TAA was administered, the LHD-fed mice and the MHD-fed mice survived longer than those fed with CD. The accumulated lipids appeared to be associated with protecting the liver against TAA toxicity (200mg/kg). Consistently, lipid-loaded Hepa 1-6 cells showed a partial resistance to hydrogen peroxide toxicity compared to those cultured in conventional media. In conclusion, while sustained steatosis impairs liver function and leads to hazardous conditions, lipids that transiently accumulate as the result of UPR or other stimuli may exert a beneficial function in the liver at least partly through scavenging reactive oxygen species.

Wnt/β-Catenin Signaling Drives Thioacetamide-Mediated Heteroprotection Against Acetaminophen-Induced Lethal Liver Injury

Preplacement of compensatory tissue repair (CTR) by exposure to a nonlethal dose of a toxicant protects animals against a lethal dose of another toxicant. Although CTR is known to heteroprotect, the underlying molecular mechanisms are not completely known. Here, we investigated the mechanisms of heteroprotection using thioacetamide (TA): acetaminophen (APAP) heteroprotection model. Male Swiss Webster mice received a low dose of TA or distilled water (DW) vehicle 24 hours prior to a lethal dose of APAP. Liver injury, tissue repair, and promitogenic signaling were studied over a time course of 24 hours after APAP overdose to the TA- and DW-primed mice (TA + APAP and DW + APAP, respectively). Thioacetamide pretreatment afforded 100% protection against APAP overdose compared to 100% lethality in the DW + APAP-treated mice. Although hepatic Cyp2e1 was similar at the time of APAP administration, immediate activation of hepatic c-Jun N-terminal kinases (JNK) was observed in the TA + APAP-treated mice compared to its delayed activation in the DW + APAP group. In contrast to the DW + APAP group, the TA + APAP-treated mice exhibited extensive CTR, which was secondary to the timely activation of Wnt/β-catenin pathway. Our data indicate that rapid activation and appropriate termination of Wnt/β-catenin signaling and modulation of JNK activity underlie TA + APAP heteroprotection.

Hepatic Overexpression of Annexin A1 and A2 in Thioacetamide-Primed Mice Protects Them Against Acetaminophen-Induced Liver Failure and Death

Compensatory tissue repair (CTR) in thioacetamide (TA)-primed rats protects them against acetaminophen (APAP)-induced lethality. This study was aimed at investigating the mechanisms of CTR-mediated heteroprotection in mice. Male Swiss Webster mice received a priming dose of TA (40 mg/kg body weight [BW] in 10 mL distilled water [DW]/kg BW, intraperitoneally [IP]). Thioacetamide-induced liver injury, CTR, and expression of annexin A1 and A2 (ANX1 and ANX2), the endogenous inhibitors of the death protein secretory phospholipase A2 (sPLA₂), were measured over a time course of 84 hours after TA priming. Both centrilobular necrosis and CTR peaked at 36 hours after TA priming as indicated by significantly increased plasma alanine transaminase (ALT) and aspartate transaminase (AST) activities, liver histology, and proliferating cell nuclear antigen immunostaining. Thioacetamide priming resulted in the overexpression of ANX1 and ANX2 at 36 to 84 hours and 12 to 60 hours, respectively. A lethal dose of APAP (600 mg/kg BW in 10 mL 0.45% NaCl/kg BW, IP) was given at 12, 24, or 36 hours after TA-priming. Thioacetamide priming did not affect the rise in plasma ALT, AST, sPLA₂, and arachidonic acid levels seen at 2 hours after the APAP overdose. Neither these biochemical parameters nor histology suggested any escalation of hepatic injury at later time points (12 and 24 hours after APAP overdose), consistent with 100% survival of the TA + APAP-treated mice compared to DW + APAP-treated mice, which had 100% mortality. Inhibition of ANX1 and ANX2 biosynthesis using cycloheximide (40 mg/kg BW in 5 mL DW/kg BW, IP) abolished this heteroprotection. Our data indicate that hepatic overexpression of ANX1 and ANX2 inhibits APAP-induced expansion of liver injury.

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.

Osthole ameliorates hepatic fibrosis and inhibits hepatic stellate cell activation

Background

Hepatic fibrosis is a dynamic process which ultimately leads to cirrhosis in almost patients with chronic hepatic injury. However, progressive fibrosis is a reversible scarring response. Activation of hepatic stellate cells (HSCs) is the prevailing process during hepatic fibrosis. Osthole is an active component majorly contained in the fruit of Cnidium monnieri (L.) Cusson. This present study investigated the therapeutic effects of osthole on rat liver fibrosis and HSC activation.

Results

We established the thioacetamide (TAA)-model of Sprague–Dawley (SD) rats to induce hepatic fibrosis. Rats were divided into three groups: control, TAA, and TAA + osthole (10 mg/kg). In vivo, osthole significantly reduced liver injury by diminishing levels of plasma AST and ALT, improving histological architecture, decreasing collagen and α-SMA accumulation, and improving hepatic fibrosis scores. Additionally, osthole reduced the expression of fibrosis-related genes significantly. Osthole also suppressed the production of fibrosis-related cytokines and chemokines. Moreover, nuclear translocation of p65 was significantly suppressed in osthole-treated liver. Osthole also ameliorated TAA-induced injury through reducing cellular oxidation. Osthole showed inhibitory effects in inflammation-related genes and chemokines production as well. In vitro, we assessed osthole effects in activated HSCs (HSC-T6 and LX-2). Osthole attenuated TGF-β1-induced migration and invasion in HSCs. Furthermore, osthole decreased TNF-α-triggered NF-κB activities significantly. Besides, osthole alleviated TGF-β1- or ET-1-induced HSCs contractility.

Conclusions

Our study demonstrated that osthole improved TAA-caused liver injury, fibrogenesis and inflammation in rats. In addition, osthole suppressed HSCs activation in vitro significantly.

Electronic supplementary material

The online version of this article (doi:10.1186/s12929-015-0168-5) contains supplementary material, which is available to authorized users.

Mouse models of liver fibrosis mimic human liver fibrosis of different etiologies

The liver has the amazing capacity to repair itself after injury; however, the same processes that are involved in liver regeneration after acute injury can cause serious consequences during chronic liver injury. In an effort to repair damage, activated hepatic stellate cells trigger a cascade of events that lead to deposition and accumulation of extracellular matrix components causing the progressive replacement of the liver parenchyma by scar tissue, thus resulting in fibrosis. Although fibrosis occurs as a result of many chronic liver diseases, the molecular mechanisms involved depend on the underlying etiology. Since studying liver fibrosis in human subjects is complicated by many factors, mouse models of liver fibrosis that mimic the human conditions fill this void. This review summarizes the general mouse models of liver fibrosis and mouse models that mimic specific human disease conditions that result in liver fibrosis. Additionally, recent progress that has been made in understanding the molecular mechanisms involved in the fibrogenic processes of each of the human disease conditions is highlighted.

Experimental liver fibrosis research: update on animal models, legal issues and translational aspects

Liver fibrosis is defined as excessive extracellular matrix deposition and is based on complex interactions between matrix-producing hepatic stellate cells and an abundance of liver-resident and infiltrating cells. Investigation of these processes requires in vitro and in vivo experimental work in animals. However, the use of animals in translational research will be increasingly challenged, at least in countries of the European Union, because of the adoption of new animal welfare rules in 2013. These rules will create an urgent need for optimized standard operating procedures regarding animal experimentation and improved international communication in the liver fibrosis community. This review gives an update on current animal models, techniques and underlying pathomechanisms with the aim of fostering a critical discussion of the limitations and potential of up-to-date animal experimentation. We discuss potential complications in experimental liver fibrosis and provide examples of how the findings of studies in which these models are used can be translated to human disease and therapy. In this review, we want to motivate the international community to design more standardized animal models which might help to address the legally requested replacement, refinement and reduction of animals in fibrosis research.

Corn peptides protect against thioacetamide-induced hepatic fibrosis in rats

Certain bioactive peptides are reported to be able to alleviate hepatic fibrosis. Our previous work has confirmed the hepatoprotective effect of corn peptides (CPs) that are prepared from a high protein by-product, corn gluten meal, on acute liver injury in an animal model. However, the antifibrotic activity of CPs remained to be elucidated. In this study, the hepatoprotective effect of CPs on thioacetamide (TAA)-induced liver fibrosis was tested. Results showed that CPs (100 mg/kg body weight) significantly decreased the levels of alanine transaminase/aspartate transaminase, laminin, type IV collagen, and type III collagen in serum and increased the serum albumin levels and total antioxidant capacity. Additionally, with CP treatment (100 mg/kg body weight), a significant decrease was observed in the levels of malondialdehyde, nitric oxide, hydroxyproline, transforming growth factor β1, and lactate dehydrogenase activity as well as the liver index, while the activity of superoxidedismutase was significantly increased in livers. The histological and morphological analysis showed that the hepatocyte structure in CP-treated rats was superior to that of TAA-injured rats, and inflammation and fibrosis were also ameliorated. Therefore, CPs can be used as an option for prevention and adjuvant therapy of liver fibrosis.

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.

Cytochrome P450 2E1: its clinical aspects and a brief perspective on the current research scenario

Research on Cytochrome P450 2E1 (CYP2E1), a key enzyme in alcohol metabolism has been very well documented in literature. Besides the involvement of CYP2E1 in alcohol metabolism as illustrated through the studies discussed in the chapter, recent studies have thrown light on several other aspects of CYP2E1 i.e. its extrahepatic expression, its involvement in several diseases and pathophysiological conditions; and CYP2E1 mediated carcinogenesis and modulation of drug efficacy. Studies involving these interesting facets of CYP2E1 have been discussed in the chapter focusing on the recent observations or ongoing studies illustrating the crucial role of CYP2E1 in disease development and drug metabolism.

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.

Protective effects of kaerophyllin against liver fibrogenesis in rats

BACKGROUND

We previously demonstrated that kaerophyllin, a lignan, isolated from a widely used traditional Chinese herb, Bupleurum scorzonerifolium, leading to the inhibition of hepatic stellate cells (HSCs) activation in vitro. This current study evaluated the in vivo role of kaerophyllin in protecting the liver against injury and fibrogenesis caused by thioacetamide (TAA) in rats and further explored the underlying mechanisms.

MATERIALS AND METHODS

Liver fibrosis in Sprague-Dawley rats was induced by intraperitoneal injection of TAA (200 mg/kg) twice per week for 6 weeks. Animals were divided into five groups: vehicle control, TAA control, TAA + low dose kaerophyllin, TAA + high dose kaerophyllin and TAA + curcumin groups. Kaerophyllin (10 or 30 mg/kg) or curcumin (150 mg/mL) was given by gavage twice per day consecutively for 4 weeks starting 2 weeks after TAA injection. Rat HSCs were used to investigate the anti-inflammatory role of kaerophyllin against tumour necrosis factor α (TNF-α) in vitro. Peroxisome proliferator-activated receptor-γ (PPAR-γ) expression was knocked down in rat HSCs using PPAR-γ small interfering RNAs.

RESULTS

Kaerophyllin significantly protected liver from injury by reducing serum aspartate transaminase and alanine transaminase levels and by improving the histological architecture and fibrosis score. In addition, kaerophyllin suppressed inflammation by reducing the mRNA of TNF-α, interleukin-1β (IL-1β) and monocyte chemoattractant protein-1 (MCP-1) genes. In HSCs, kaerophyllin elevated PPAR-γ activity and reduced TNF-α-stimulated mRNA levels of intracellular adhesion molecule-1 (ICAM-1), MCP-1 and IL-1β genes, which were reversed by small interfering RNA knockdown of PPAR-γ gene.

CONCLUSIONS

Our results demonstrated that kaerophyllin protected the rat liver from TAA-caused injury and fibrogenesis by suppressing hepatic inflammation and inhibiting HSC activation, possibly through upregulation of PPAR-γ expression.

Once initiated, how does toxic tissue injury expand?

Once initiated, how tissue injury expands after high toxicant doses, even after their complete elimination, is not understood. Free-radical generation was initially proposed to mediate progression of injury. However, mechanisms proposed thus far have remained unsubstantiated. Necrotic injury is characterized by loss of osmoregulation, cell swelling, blebbing, and cell rupture. This exposes cytosolic enzymes, including proteases, phospholipases, and lysosomal Ca(2+)-dependent enzymes, to high extracellular calcium (Ca(2+)). Activated hydrolytic enzymes, termed 'death proteins,' hydrolyze their substrates in the plasma membrane of neighboring cells, commencing self-perpetuated injury progression. Likewise, ischemia-reperfusion injury exposes the hydrolytic enzymes to high Ca(2+), fuelling the progression of tissue injury. This mechanism is independent of the offending toxicant that initiates the injury. I present here a case for therapeutic intervention with inhibitors directed against death proteins as a means to avert organ failure and death well after the poisoning event.

Curcumin protects against thioacetamide-induced hepatic fibrosis by attenuating the inflammatory response and inducing apoptosis of damaged hepatocytes

Inflammation and hepatic stellate cell (HSC) activation are the most crucial steps in the formation of hepatic fibrosis. Hepatocytes damaged by viral or bacterial infection, alcohol or toxic chemicals initiate an inflammatory response that activates collagen production by HSCs. Recent studies indicate curcumin has liver-protective effects due to its anti-inflammatory, antioxidant and anticancer activities; however, the mechanisms are not well understood. In this study, we show that curcumin protected against hepatic fibrosis in BALB/c mice in vivo by inhibiting HSC activation, inflammatory responses and inducing apoptosis of damaged hepatocytes. Using the thioacetamide (TAA)-induced hepatic fibrosis animal model, we found that curcumin treatment up-regulated P53 protein expression and Bax messenger RNA (mRNA) expression and down-regulated Bcl-2 mRNA expression. Together, these responses increased hepatocyte sensitivity to TAA-induced cytotoxicity and forced the damaged cells to undergo apoptosis. Enhancing the tendency of damaged hepatocytes to undergo apoptosis may be the protective mechanism whereby curcumin suppresses inflammatory responses and hepatic fibrogenesis. These results provide a novel insight into the cause of hepatic fibrosis and the cytoprotective effects curcumin has on hepatic fibrosis suppression.

Efficacy of urine bile acid as a non-invasive indicator of liver damage in rats

Estimation of liver damage is important in the pathophysiological and toxicological study of liver disease. As a novel, non-invasive marker of liver damage, we studied the efficacy of urine bile acids (UBA) in a rat model of liver disease. Thioacetamide (TAA)-treated rats were used in this study. Single intraperitoneal administration of high-dose TAA induces severe damage to the liver, and thus is used as a model of acute hepatitis. Continuous administration of low-dose TAA yields mild damage to the liver, and induces cirrhosis and hepatic tumors. In this study, it was found that both acute and chronic administration of TAA was associated with a dose-dependent elevation of UBA. The elevation of UBA content correlated with the alteration of blood biochemical indicators, and UBA screening showed a remarkable ability to distinguish liver-damaged rats from healthy rats. In particular, UBA analysis was found to have high sensitivity, specificity, and positive predictive value for the screening of rats with abnormal serum alkaline phosphatase (ALP) activity due to chronic liver damage, which was confirmed to include cholestasis and subsequent cirrhosis by liver histological analysis. In conclusion, we demonstrated that measurement of UBA is a simple, non-invasive and effective method for the screening of cholestasis in TAA-treated rats. We suggest that UBA analysis may have potent applicability for monitoring the progress of liver damage in animal models of chronic liver disease, such as cirrhosis and hepatic encephalopathy.

CYP2E1 and oxidative liver injury by alcohol

Ethanol-induced oxidative stress seems to play a major role in mechanisms by which ethanol causes liver injury. Many pathways have been suggested to contribute to the ability of ethanol to induce a state of oxidative stress. One central pathway seems to be the induction of cytochrome P450 2E1 (CYP2E1) by ethanol. CYP2E1 metabolizes and activates many toxicological substrates, including ethanol, to more reactive, toxic products. Levels of CYP2E1 are elevated under a variety of physiological and pathophysiological conditions and after acute and chronic alcohol treatment. CYP2E1 is also an effective generator of reactive oxygen species such as the superoxide anion radical and hydrogen peroxide and, in the presence of iron catalysts, produces powerful oxidants such as the hydroxyl radical. This review article summarizes some of the biochemical and toxicological properties of CYP2E1 and briefly describes the use of cell lines developed to constitutively express CYP2E1 and CYP2E1 knockout mice in assessing the actions of CYP2E1. Possible therapeutic implications for treatment of alcoholic liver injury by inhibition of CYP2E1 or CYP2E1-dependent oxidative stress will be discussed, followed by some future directions which may help us to understand the actions of CYP2E1 and its role in alcoholic liver injury.

S-Adenosylmethionine Exerts a Protective Effect against Thioacetamide-induced Injury in Primary Cultures of Rat Hepatocytes

S-adenosylmethionine (SAMe) has been shown to protect hepatocytes from toxic injury, both experimentally-induced in animals and in isolated hepatocytes. The mechanisms by which SAMe protects hepatocytes from injury can result from the pathways of SAMe metabolism. Unfortunately, data documenting the protective effect of SAMe against mitochondrial damage from toxic injury are not widely available. Thioacetamide is frequently-used as a model hepatotoxin, which causes in vivo centrilobular necrosis. Even though thioacetamide-induced liver necrosis in rats was alleviated by SAMe, the mechanisms of this protective effect remain to be verified. The aim of our study was to determine the protective mechanisms of SAMe on thioacetamide-induced hepatocyte injury by using primary hepatocyte cultures. The release of lactate dehydrogenase (LDH) from cells incubated with thioacetamide for 24 hours, was lowered by simultaneous treatment with SAMe, in a dose-dependent manner. The inhibitory effect of SAMe on thioacetamide-induced lipid peroxidation paralleled the effect on cytotoxicity. A decrease in the mitochondrial membrane potential, as determined by Rhodamine 123 accumulation, was also prevented. The attenuation by SAMe of thioacetamide-induced glutathione depletion was determined after subsequent incubation periods of 48 and 72 hours. SAMe protects both cytoplasmic and mitochondrial membranes. This effect was more pronounced during the development of thioacetamide-induced hepatocyte injury that was mediated by lipid peroxidation. Continuation of the SAMe treatment then led to a reduction in glutathione depletion, as a potential consequence of an increase in glutathione production, for which SAMe is a precursor.

Impact of repeated exposure on toxicity of perchloroethylene in Swiss Webster mice

The aim was to study the subchronic toxicity of perchloroethylene (Perc) by measuring injury and repair in liver and kidney in relation to disposition of Perc and its major metabolites. Male SW mice (25-29g) were given three dose levels of Perc (150, 500, and 1000 mg/kg day) via aqueous gavage for 30 days. Tissue injury was measured during the dosing regimen (0, 1, 7, 14, and 30 days) and over a time course of 24-96h after the last dose (30 days). Perc produced significant liver injury (ALT) after single day exposure to all three doses. Liver injury was mild to moderate and regressed following repeated exposure for 30 days. Subchronic Perc exposure induced neither kidney injury nor dysfunction during the entire time course as evidenced by normal renal histology and BUN. TCA was the major metabolite detected in blood, liver, and kidney. Traces of DCA were also detected in blood at initial time points after single day exposure. With single day exposure, metabolism of Perc to TCA was saturated with all three doses. AUC/dose ratio for TCA was significantly decreased with a concomitant increase in AUC/dose of Perc levels in liver and kidney after 30 days as compared to 1 day exposures, indicating inhibition of metabolism upon repeated exposure to Perc. Hepatic CYP2E1 expression and activity were unchanged indicating that CYP2E1 is not the critical enzyme inhibited. Hepatic CYP4A expression, measured as a marker of peroxisome proliferation was increased transiently only on day 7 with the high dose, but was unchanged at later time points. Liver tissue repair peaked at 7 days, with all three doses and was sustained after medium and high dose exposure for 14 days. These data indicate that subchronic Perc exposure via aqueous gavage does not induce nephrotoxicity and sustained hepatotoxicity suggesting adaptive hepatic repair mechanisms. Enzymes other than CYP2E1, involved in the metabolism of Perc may play a critical role in the metabolism of Perc upon subchronic exposure in SW mice. Liver injury decreased during repeated exposure due to inhibition of metabolism and possibly due to adaptive tissue repair mechanisms.

Toxicokinetics and toxicity of thioacetamide sulfoxide: a metabolite of thioacetamide

Thioacetamide (TA) is bioactivated by CYP2E1 to TA sulfoxide (TASO), and to the highly reactive sulfdioxide (TASO(2)), which initiates hepatic necrosis by covalent binding. Previously, we have established that TA exhibits saturation toxicokinetics over a 12-fold dose range, which explains the lack of dose-response for bioactivation-based liver injury. In vivo and in vitro studies indicated that the second step (TASO-->TASO(2)) of TA bioactivation is less efficient than the first one (TA-->TASO). The objective of the present study was to specifically test the saturation of the second step of TA bioactivation by directly administering TASO, which obviates the contribution from first step, i.e. TA-->TASO. Male SD rats were injected with low (50mg/kg, ip), medium (100mg/kg) and high (LD(70), 200mg/kg) doses of TASO. Bioactivation-mediated liver injury that occurs in the initial time points (6 and 12h), estimated by plasma ALT, AST and liver histopathology over a time course, was not dose-proportional. Escalation of liver injury thereafter was dose dependent: low dose injury subsided; medium dose injury escalated upto 36h before declining; high dose injury escalated from 24h leading to 70% mortality. TASO was quantified in plasma by HPLC at various time points after administration of the three doses. With increasing dose (i.e., from 50 to 200mg/kg), area under the curve (AUC) and C(max) increased more than dose proportionately, indicating that TASO bioactivation exhibits saturable kinetics. Toxicokinetics and initiation of liver injury of TASO are similar to that of TA, although TASO-initiated injury occurs at lower doses. These findings indicate that bioactivation of TASO to its reactive metabolite is saturable in the rat as suggested by previous studies with TA.