Role of CYP3A in ethanol-mediated increases in acetaminophen hepatotoxicity

In silico Identification and Mechanism Exploration of Hepatotoxic Ingredients in Traditional Chinese Medicine

BACKGROUNDS AND AIMS

Recently, a growing number of hepatotoxicity cases aroused by Traditional Chinese Medicine (TCM) have been reported, causing increasing concern. To date, the reported predictive models for drug induced liver injury show low prediction accuracy and there are still no related reports for hepatotoxicity evaluation of TCM systematically. Additionally, the mechanism of herb induced liver injury (HILI) still remains unknown. The aim of the study was to identify potential hepatotoxic ingredients in TCM and explore the molecular mechanism of TCM against HILI.

MATERIALS AND METHODS

In this study, we developed consensus models for HILI prediction by integrating the best single classifiers. The consensus model with best performance was applied to identify the potential hepatotoxic ingredients from the Traditional Chinese Medicine Systems Pharmacology database (TCMSP). Systems pharmacology analyses, including multiple network construction and KEGG pathway enrichment, were performed to further explore the hepatotoxicity mechanism of TCM.

RESULTS

16 single classifiers were built by combining four machine learning methods with four different sets of fingerprints. After systematic evaluation, the best four single classifiers were selected, which achieved a Matthews correlation coefficient (MCC) value of 0.702, 0.691, 0.659, and 0.717, respectively. To improve the predictive capacity of single models, consensus prediction method was used to integrate the best four single classifiers. Results showed that the consensus model C-3 (MCC = 0.78) outperformed the four single classifiers and other consensus models. Subsequently, 5,666 potential hepatotoxic compounds were identified by C-3 model. We integrated the top 10 hepatotoxic herbs and discussed the hepatotoxicity mechanism of TCM via systems pharmacology approach. Finally, Chaihu was selected as the case study for exploring the molecular mechanism of hepatotoxicity.

CONCLUSION

Overall, this study provides a high accurate approach to predict HILI and an in silico perspective into understanding the hepatotoxicity mechanism of TCM, which might facilitate the discovery and development of new drugs.

Carbamazepine-Induced Liver Injury Requires CYP3A-Mediated Metabolism and Glutathione Depletion in Rats

Carbamazepine (CBZ) is widely used as an antiepileptic agent and causes rare but severe liver injury in humans. It has been generally recognized that reactive metabolites formed via the metabolic activation reaction contribute to the onset of liver injuries by several drugs. However, the role of CBZ metabolism in the development of liver injury is not fully understood. In this study, we developed a novel rat model of CBZ-induced liver injury and attempted to elucidate the associated mechanisms by focusing on the metabolism of CBZ. The repeated administration of CBZ for 5 days in combination with l-buthionine sulfoximine (BSO), a glutathione (GSH) synthesis inhibitor, resulted in increases in the plasma alanine aminotransferase (ALT) levels and centrilobular necrosis in the liver that were observed in various degrees. The CBZ and 2-hydroxy-CBZ concentrations in the plasma after the last CBZ administration were lower in the rats with high plasma ALT levels compared with those with normal plasma ALT levels, showing the possibility that the further metabolism of CBZ and/or 2-hydroxy-CBZ is associated with the liver injury. Although a single administration of CBZ did not affect the plasma ALT levels, even when cotreated with BSO, pretreatment with dexamethasone, a CYP3A inducer, increased the plasma ALT levels. In addition, the rats cotreated with troleandomycin or ketoconazole, CYP3A inhibitors, suppressed the increased plasma ALT levels. In conclusion, reactive metabolite(s) of CBZ produced by CYP3A under the GSH-depleted condition might be involved in the development of liver injury in rats.

Identification and characterization of in vitro and in vivo metabolites of steroidal alkaloid veratramine

Veratramine, a steroidal alkaloid originating from Veratrum nigrum L., has demonstrated distinct anti-tumor and anti-hypertension effects, however, its metabolism has rarely been explored. The objective of the current study was to provide a comprehensive investigation of its metabolic pathways. The in vitro metabolic profiles of veratramine were evaluated by incubating it with liver microsomes and cytosols. The in vivo metabolic profiles in plasma, bile, urine and feces were monitored by UPLC-MS/MS after oral (20 mg/kg) and i.v. (50 µg/kg) administration in rats. Meanwhile, related P450s inhibitors and recombinant P450s and SULTs were used to identify the isozymes responsible for its metabolism. Eleven metabolites of veratramine, including seven hydroxylated, two sulfated and two glucuronidated metabolites, were characterized. Unlike most alkaloids, the major reactive sites of veratramine were on ring A and B instead of on the amine moiety. CYP2D6 was the major isozyme mediating hydroxylation, and substrate inhibition was observed with a Vmax , Ki and Clint of 2.05 ± 0.53 nmol/min/mg, 33.08 ± 10.13 µ m and 13.58 ± 1.27 µL/min/mg. SULT2A1, with Km , Vmax and Clint values of 19.37 ± 0.87 µ m, 1.51 ± 0.02 nmol/min/mg and 78.19 ± 8.57 µL/min/mg, was identified as the major isozyme contributing to its sulfation. In conclusion, CYP2D6 and SULT2A1 mediating hydroxylation and sulfation were identified as the major biotransformation for veratramine.

Cold Storage of Rat Hepatocyte Spheroids

Cell-based therapies for liver disease rely on a high-quality supply of hepatocytes and a means for storage during transportation from site of isolation to site of usage. Unfortunately, frozen cryopreservation is associated with unacceptable loss of hepatocyte viability after thawing. The purpose of this study was to optimize conditions for cold storage of rat hepatocyte spheroids without freezing. Rat hepatocytes were isolated by a two-step perfusion method; hepatocyte spheroids were formed during 48 h of rocked culture in serum-free medium (SFM). Spheroids were then maintained in rocked culture at 37°C (control condition) or cold stored at 4°C for 24 or 48 h in six different cold storage solutions: SFM alone; SFM + 1 mM deferoxamine (Def); SFM + 1 μM cyclosporin A (CsA); SFM + 1 mM Def + 1 μM CsA, University of Wisconsin (UW) solution alone, UW + 1 mM Def. Performance metrics after cold storage included viability, gene expression, albumin production, and functional activity of cytochrome P450 enzymes and urea cycle proteins. We observed that cold-induced injury was reduced significantly by the addition of the iron chelator (Def) to both SFM and UW solution. Performance metrics (ammonia detoxification, albumin production) of rat hepatocyte spheroids stored in SFM + Def for 24 h were significantly increased from SFM alone and approached those in control conditions, while performance metrics after cold storage in SFM alone or cold storage for 48 h were both significantly reduced. A serum-free medium supplemented with Def allowed hepatocyte spheroids to tolerate 24 h of cold storage with less than 10% loss in viability and functionality. Further research is warranted to optimize a solution for extended cold storage of hepatocyte spheroids.

Drug-induced liver injury: insights from genetic studies

Drug-induced liver injury (DILI) is an increasing health problem and a challenge for physicians, regulatory bodies and the pharmaceutical industry, not only because of its potential severity and elusive pathogenesis but also because it is often inaccurately diagnosed, commonly missed entirely and more often not reported. The general view is that idiosyncratic DILI, which is not predictable whether based on the pharmacology of the drug or on the dose administered, is determined by the presence in the recipient of variants in, or expression of, genes coding for key metabolic pathways and/or the immune response, and the interaction of these genetic variants with environmental variables. Furthermore, idiosyncratic DILI is an example of a complex-trait disease with two or more susceptibility loci, as reflected by the frequency of genetic variants in the population often being higher than the occurrence of significant liver injury. Polymorphisms of bioactivation/toxification pathways via the CYP450 enzymes (Phase I), detoxification reactions (Phase II) and excretion/transport (Phase III), together with immunological factors that might determine DILI are reviewed. Challenges such as gene-trait association studies and whole-genome studies, and future approaches to the study of DILI are explored. Better knowledge of the candidate genes involved could provide further insight for the prospective identification of susceptible patients at risk of developing drug-induced hepatotoxicity, development of new diagnostic tools and new treatment strategies with safer drugs.

Acetaminophen bioactivation by human cytochrome P450 enzymes and animal microsomes

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

Ethanol intoxication increases hepatic N-lysyl protein acetylation

The acetylation of the epsilon-amino group of lysine to form N-acetyl lysine (N-AcLys)-modified proteins regulates the activity of metabolic proteins. Because of the multiple effects of ethanol upon hepatic metabolism, it was hypothesized that ethanol exposure increases the hepatic content of N-AcLys-modified proteins. To test this hypothesis, rats or mice were exposed to ethanol using a liquid diet regimen. Content of N-AcLys-modified proteins was elevated more than 5-fold after 6 weeks of ethanol exposure and persisted after ethanol withdrawal. Use of CYP2E1-knockout mice demonstrated that ethanol-induced acetylation was not dependent solely on CYP2E1 expression. The mitochondrial content of N-AcLys-modified proteins was elevated almost 5-fold following 6 weeks of ethanol exposure. Mitochondrial content of the deacetylase Sirt3 was unchanged by 6 weeks of ethanol exposure. These data indicate ethanol intoxication changes the acetylation status of, and likely the activity of, multiple mitochondrial proteins.

Role of biotransformation in 3-(3,5-dichlorophenyl)-2,4-thiazolidinedione-induced hepatotoxicity in Fischer 344 rats

Cytochrome P450 (CYP)-mediated metabolism in the thiazolidinedione (TZD) ring may contribute to the hepatotoxicity of the insulin-sensitizing agents such as troglitazone. We were interested in determining if biotransformation could also be a factor in the liver damage associated with another TZD ring containing compound, 3-(3,5-dichlorophenyl)-2,4-thiazolidinedione (DCPT). Therefore, hepatotoxic doses of DCPT (0.6 or 1.0 mmol/kg, i.p.) were administered to male Fischer 344 rats after pretreatment with vehicle, 1-aminobenzotriazole (ABT, non-selective CYP inhibitor) and troleandomycin (TAO, CYP3A inhibitor). Alternatively, rats were pretreated with vehicle or the CYP3A inducer dexamethasone (DEX) prior to a non-toxic DCPT dose (0.2 mmol/kg, i.p.). Vehicle-, ABT-, TAO- and DEX-only control groups were also run. Toxicity was assessed 24 h after DCPT administration. Both hepatotoxic doses of DCPT induced elevations in serum alanine aminotransferase (ALT) levels that were attenuated by ABT or TAO pretreatment. Liver sections from rats that received vehicle+DCPT revealed areas of gross necrosis and neutrophil invasion, whereas sections from ABT+DCPT and TAO+DCPT rats showed minor changes compared to controls. DEX pretreatment potentiated ALT levels associated with the non-toxic DCPT dose. Furthermore, DEX+DCPT rat liver sections exhibited hepatic injury when compared against rats that received vehicle+DCPT. Blood urea nitrogen levels, urinalysis and kidney morphology were not markedly altered by any combination of pretreatments or treatments. Enzyme activity and Western blotting experiments with rat liver microsomes confirmed the effects of the various pretreatments. Our results suggest that hepatic CYP3A isozymes may be involved in DCPT-induced liver damage in male rats. We believe this is the first report demonstrating that modulation of the biotransformation of a TZD ring-containing compound can alter hepatotoxicity in a common animal model.

Genomic profiling in nuclear receptor-mediated toxicity

Nuclear receptors (NRs) are attractive drug targets due to their role in regulation of a wide range of physiologic responses. In addition to providing therapeutic value, many pharmaceutical agents along with environmental chemicals are ligands for NRs and can cause adverse health effects that are directly related to activation of NRs. Identifying the molecular events that produce a toxic response may be confounded by the fact that there is a significant overlap in the biological processes that NRs regulate. Microarrays and other methods for gene expression profiling have served as useful, sensitive tools for discerning the mechanisms by which therapeutics and environmental chemicals invoke toxic effects. The capability to probe thousands of genes simultaneously has made genomics a prime technology for identifying drug targets, biomarkers of exposure/toxicity and key players in the mechanisms of disease. The complex intertwining networks regulated by NRs are hard to probe comprehensively without global approaches and genomics has become a key technology that facilitates our understanding of NR-dependent and -independent events. The future of drug discovery, design and optimization, and risk assessment of chemical toxicants that activate NRs will inevitably involve genomic profiling. This review will focus on genomics studies related to PPAR, CAR, PXR, RXR, LXR, FXR, and AHR.

Role of CYP3A and CYP2E1 in alcohol-mediated increases in acetaminophen hepatotoxicity: comparison of wild-type and Cyp2e1(-/-) mice

CYP2E1 is widely accepted as the sole form of cytochrome P450 responsible for alcohol-mediated increases in acetaminophen (APAP) hepatotoxicity. However, we previously found that alcohol [ethanol and isopentanol (EIP)] causes increases in APAP hepatotoxicity in Cyp2e1(-/-) mice, indicating that CYP2E1 is not essential. Here, using wild-type and Cyp2e1(-/-) mice, we investigated the relative roles of CYP2E1 and CYP3A in EIP-mediated increases in APAP hepatotoxicity. We found that EIP-mediated increases in APAP hepatotoxicity occurred at lower APAP doses in wild-type mice (300 mg/kg) than in Cyp2e1(-/-) mice (600 mg/kg). Although this result suggests that CYP2E1 has a role in the different susceptibilities of these mouse lines, our findings that EIP-mediated increases in CYP3A activities were greater in wild-type mice compared with Cyp2e1(-/-) mice raises the possibility that differential increases in CYP3A may also contribute to the greater APAP sensitivity in EIP-pretreated wild-type mice. At the time of APAP administration, which followed an 11 h withdrawal from the alcohols, alcohol-induced levels of CYP3A were sustained in both mouse lines, whereas CYP2E1 was decreased to constitutive levels in wild-type mice. The CYP3A inhibitor triacetyloleandomycin (TAO) decreased APAP hepatotoxicity in EIP-pretreated wild-type and Cyp2e1(-/-) mice. TAO treatment in vivo resulted in inhibition of microsomal CYP3A-catalyzed activity, measured in vitro, with no inhibition of CYP1A2 and CYP2E1 activities. In conclusion, these findings suggest that both CYP3A and CYP2E1 contribute to APAP hepatotoxicity in alcohol-treated mice.

Gene expression profile in liver of differing ages of rats after single oral administration of acetaminophen

In order to verify the influence of the rat age on hepatotoxicity, male Sprague-Dawley rats of 6 (young) and 12 (adult) weeks of age were orally administered acetaminophen (APAP), isoniazid (INH), or carbon tetrachloride (CCl4). Liver samples were obtained in a time-course manner, and changes in gene expression examined by an Affymetrix GeneChip. APAP caused more prominent hepatic injury with respect to pathology and blood biochemistry in adults than in young rats, whereas no obvious age-related differences were observed in INH- or CCl4-treated rats. Comparing gene expression in control rats, CYP3A13 was higher and GSTY2c was lower in adults, suggesting that production of the active metabolite of APAP is higher and its detoxification is lower in adults. The total amount of glutathione and total SH in rat liver was found to be higher in adult rats whereas the extent of its reduction by APAP was larger in adults. A detailed analysis of genes showing age-related differences revealed that some of them were different not in their extent but in their time course, i.e., the stress responses occurred earlier in the young than in the adult, resulting in a difference at 24 hr after dosing. These results suggest that the age-related difference in toxicity would be attributed to a higher expression of CYP3A13, producing the active metabolite of APAP as well as the lower expression of the detoxification enzyme, GSTY2c, in adult rats. Furthermore, these differences affect the time course of APAP toxicity. The present study clearly depicts the advantage of the multi-time, multi-dose protocol employed in our project for analyzing the mechanism of toxicity by gene expression profiling.

3D hepatocyte monolayer on hybrid RGD/galactose substratum

Hepatocyte-based applications such as xenobiotics metabolism and toxicity studies usually require hepatocytes anchoring onto flat substrata that support their functional maintenance. Conventional cell culture plates coated with natural matrices or synthetic ligands allow hepatocytes to adhere tightly as two-dimensional (2D) monolayer but these tightly anchored hepatocytes rapidly lose their differentiated functions. On galactosylated substrata, hepatocytes adhere loosely; and readily form three-dimensional (3D) spheroids that can maintain high levels of cellular functions. These spheroids detach easily from the substrata and exhibit poor mass transport properties unsuitable for many applications. Here, we have developed a hybrid RGD/galactose substratum based on polyethylene terephthalate film conjugated with both RGD peptide and galactose ligand to enhance cell adhesion and functions synergistically. Primary hepatocytes adhere effectively onto the transparent hybrid substratum in 96-well plates as monolayer while exhibiting high levels of liver-specific functions, morphology and cell-cell interactions typically seen in the 3D hepatocyte spheroids. The hepatocytes cultured onto the hybrid substratum also exhibit high levels of sensitivity to a model drug acetaminophen similar to the 3D hepatocyte spheroids. The monolayer of hepatocytes exhibiting the 3D cell behaviors on this flat hybrid substratum can be useful for various applications requiring both effective mass transfer and cellular support.

Ginkgolide A contributes to the potentiation of acetaminophen toxicity by Ginkgo biloba extract in primary cultures of rat hepatocytes

The present cell culture study investigated the effect of Ginkgo biloba extract pretreatment on acetaminophen toxicity and assessed the role of ginkgolide A and cytochrome P450 3A (CYP3A) in hepatocytes isolated from adult male Long-Evans rats provided ad libitum with a standard diet. Acetaminophen (7.5-25 mM for 24 h) conferred hepatocyte toxicity, as determined by the lactate dehydrogenase (LDH) assay. G. biloba extract alone increased LDH leakage in hepatocytes at concentrations > or =75 mug/ml and > or =750 mug/ml after a 72 h and 24 h treatment period, respectively. G. biloba extract (25 or 50 mug/ml once every 24 h for 72 h) potentiated LDH leakage by acetaminophen (10 mM for 24 h; added at 48 h after initiation of extract pretreatment). The effect was confirmed by a decrease in [(14)C]-leucine incorporation. At the level present in a modulating concentration (50 mug/ml) of the extract, ginkgolide A (0.55 mug/ml), which increased CYP3A23 mRNA levels and CYP3A-mediated enzyme activity, accounted for part but not all of the potentiating effect of the extract on acetaminophen toxicity. This occurred as a result of CYP3A induction by ginkgolide A because triacetyloleandomycin (TAO), a specific inhibitor of CYP3A catalytic activity, completely blocked the effect of ginkgolide A. Ginkgolide B, ginkgolide C, ginkgolide J, quercetin, kaempferol, isorhamnetin, and isorhamnetin-3-O-rutinoside did not alter the extent of LDH leakage by acetaminophen. In summary, G. biloba pretreatment potentiated acetaminophen toxicity in cultured rat hepatocytes and ginkgolide A contributed to this novel effect of the extract by inducing CYP3A.

Involvement of Toll-like receptor 4 in acetaminophen hepatotoxicity

The objective of this study was to determine whether Toll-like receptor 4 (TLR4) has a role in alcohol-mediated acetaminophen (APAP) hepatotoxicity. TLR4 is involved in the inflammatory response to endotoxin. Others have found that ethanol-mediated liver disease is decreased in C3H/HeJ mice, which have a mutated TLR4 resulting in a decreased response to endotoxin compared with endotoxin-responsive mice. In the present study, short-term (1 wk) pretreatment with ethanol plus isopentanol, the predominant alcohols in alcoholic beverages, caused no histologically observed liver damage in either C3H/HeJ mice or endotoxin-responsive C3H/HeN mice, despite an increase in nitrotyrosine levels in the livers of C3H/HeN mice. In C3H/HeN mice pretreated with the alcohols, subsequent exposure to APAP caused a transient decrease in liver nitrotyrosine formation, possibly due to competitive interaction of peroxynitrite with APAP producing 3-nitroacetaminophen. Treatment with APAP alone resulted in steatosis in addition to congestion and necrosis in both C3H/HeN and C3H/HeJ mice, but the effects were more severe in endotoxin-responsive C3H/HeN mice. In alcohol-pretreated endotoxin-responsive C3H/HeN mice, subsequent exposure to APAP resulted in further increases in liver damage, including severe steatosis, associated with elevated plasma levels of TNF-alpha. In contrast, alcohol pretreatment of C3H/HeJ mice caused little to no increase in APAP hepatotoxicity and no increase in plasma TNF-alpha. Portal blood endotoxin levels were very low and were not detectably elevated by any of the treatments. In conclusion, this study implicates a role of TLR4 in APAP-mediated hepatotoxicity.

Role of the nuclear receptor pregnane X receptor in acetaminophen hepatotoxicity

The pregnane X receptor (PXR) is a transcriptional regulator of xenobiotic metabolizing enzymes, including cytochrome P450 3A (CYP3A), and transporters. Pretreatment of mice and rats with inducers of CYP3A increases acetaminophen (APAP) hepatotoxicity. In untreated mice, the amount of hepatic CYP3A11 mRNA is 4-fold greater in PXR(-/-) mice compared to wild-type mice (Guo et al., 2003), a finding anticipated to increase APAP hepatotoxicity in PXR(-/-) mice. We investigated APAP hepatotoxicity in wild-type and PXR(-/-) mice in a C57BL/6 background, with APAP administered by gavage. Despite a 2.5-fold higher level of total hepatic CYP3A protein and a 3.6-fold higher level of CYP3A activity compared to wild-type mice, PXR(-/-) mice were less sensitive to APAP hepatotoxicity. Hepatic levels of CYP2E1 were identical in the two mouse lines, but hepatic CYP1A2 levels were 3-fold greater in wild-type mice compared to PXR(-/-) mice. Caffeine, an inhibitor of CYP1A2 activity and an enhancer of CYP3A activity, decreased APAP hepatotoxicity in wild-type mice. APAP uptake was 1.5-fold greater in wild-type mice compared to PXR(-/-) mice. No significant differences in the formation of APAP glucuronide and sulfate-conjugated metabolites were observed between wild-type and PXR(-/-) mice. Glutathione levels were similar in the two mouse lines and were transiently decreased to similar amounts after APAP administration. Our finding that APAP hepatotoxicity was decreased in PXR(-/-) mice indicates that PXR is an important modulator of APAP hepatotoxicity, through positive modulation of constitutive CYP1A2 expression and possibly through increased APAP absorption.

Chronic ethanol exposure downregulates hepatic expression of pregnane X receptor and P450 3A11 in female ICR mice

Pregnane X receptor (PXR) is a nuclear receptor that regulates cytochrome P450 3A (CYP3A) gene transcription in a ligand-dependent manner. Ethanol has been reported to be either an inducer or an inhibitor of CYP3A expression. In this study, we investigated the effects of chronic ethanol exposure on PXR and P450 3A11 gene expression in mouse liver. Female ICR mice were administered by gavage with different doses (1000, 2000 and 4000 mg/kg) of ethanol for up to 5 weeks. Hepatic PXR and P450 3A11 mRNA levels were measured using RT-PCR. Erythromycin N-demethylase (ERND) activity was used as an indicator of CYP3A protein expression. Results showed that chronic ethanol exposure markedly decreased hepatic PXR and P450 3A11 mRNA levels. Consistent with downregulation of P450 3A11 mRNA, chronic ethanol exposure significantly decreased ERND activity in a dose-dependent manner. Additional experiment showed that chronic ethanol exposure significantly increased plasma endotoxin level and hepatic CD14 and TLR-4 mRNA expression, all of which were blocked by elimination of Gram-negative bacteria and endotoxin with antibiotics. Correspondingly, pretreatment with antibiotics reversed the downregulation of PXR and P450 3A11 mRNA expression and ERND activity in mouse liver. Furthermore, the downregulation of hepatic PXR and P450 3A11 mRNA expression was significantly attenuated in mice pretreated with GdCl(3), a selective Kupffer cell toxicant. GdCl(3) pretreatment also significantly attenuated chronically ethanol-induced decrease in ERND activity. These results indicated that activation of Kupffer cells by gut-derived endotoxin contributes to downregulation of hepatic PXR and P450 3A11 expression during chronic alcohol intoxication.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Human and animal hepatocytes in vitro with extrapolation in vivo

Human and animal hepatocytes are now being used as an in vitro technique to aid drug discovery by predicting the in vivo metabolic pathways of drugs or new chemical entities (NCEs), identifying drug-metabolizing enzymes and predicting their in vivo induction. Because of the difficulty of establishing whether the cytotoxic susceptibility of human hepatocytes to xenobiotics/drugs in vitro could be used to predict in vivo human hepatotoxicity, a comparison of the susceptibility of the hepatocytes of human and animal models to six chemical classes of drugs/xenobiotics in vitro have been related to their in vivo hepatotoxicity and the corresponding activity of their metabolizing enzymes. This study showed that the cytotoxic effectiveness of 16 halobenzenes towards rat hepatocytes in vitro using higher doses and short incubation times correlated well with rat hepatotoxic effectiveness in vivo with lower doses/longer times. The hepatic/hepatocyte xenobiotic metabolizing enzyme activities of various animal species and human have been reviewed for use by veterinarians and research scientists. Where possible, recommendations have been made regarding which animal hepatocyte model is most applicable for modeling the susceptibility to xenobiotic induced hepatotoxicity of those humans with slow versus rapid metabolizing enzyme polymorphisms. These recommendations are based on the best human fit for animal drug/xenobiotic metabolizing enzymes in terms of activity, kinetics and substrate/inhibitor specificity. The use of human hepatocytes from slow versus rapid metabolizing individuals for drug metabolism/cytotoxicity studies; and the research use of freshly isolated rat hepatocytes and "Accelerated Cytotoxicity Mechanism Screening" (ACMS) techniques for identifying drug/xenobiotic reactive metabolites are also described. Using these techniques the molecular hepatocytotoxic mechanisms found in vitro for seven classes of xenobiotics/drugs were found to be similar to the rat hepatotoxic mechanisms reported in vivo.

The cyp2e1-humanized transgenic mouse: role of cyp2e1 in acetaminophen hepatotoxicity

The cytochrome P450 (P450) CYP2E1 enzyme metabolizes and activates a wide array of toxicological substrates, including alcohols, the widely used analgesic acetaminophen, acetone, benzene, halothane, and carcinogens such as azoxymethane and dimethylhydrazine. Most studies on the biochemical and pharmacological actions of CYP2E1 are derived from studies with rodents, rabbits, and cultured hepatocytes; therefore, extrapolation of the results to humans can be difficult. Creating "humanized" mice by introducing the human CYP2E1 gene into Cyp2e1-null mice can circumvent this disadvantage. A transgenic mouse line expressing the human CYP2E1 gene was established. Western blot and high-performance liquid chromatography/mass spectrometry analyses revealed human CYP2E1 protein expression and enzymatic activity in the liver of CYP2E1-humanized mice. Treatment of mice with the CYP2E1 inducer acetone demonstrated that human CYP2E1 was inducible in this transgenic model. The response to the CYP2E1 substrate acetaminophen was explored in the CYP2E1-humanized mice. Hepatotoxicity, resulting from the CYP2E1-mediated activation of acetaminophen, was demonstrated in the livers of CYP2E1-humanized mice by elevated serum alanine aminotransferase levels, increased hepatocyte necrosis, and decreased P450 levels. These data establish that in this humanized mouse model, human CYP2E1 is functional and can metabolize and activate different CYP2E1 substrates such as chlorzoxazone, p-nitrophenol, acetaminophen, and acetone. CYP2E1-humanized mice will be of great value for delineating the role of human CYP2E1 in ethanol-induced oxidative stress and alcoholic liver damage. They will also function as an important in vivo tool for predicting drug metabolism and disposition and drug-drug interactions of chemicals that are substrates for human CYP2E1.

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

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

Induction of CYP3A by ethanol in multiple in vitro and in vivo models

BACKGROUND

Cytochrome P-450 3A (CYP3A) is responsible for the metabolism of numerous therapeutic agents. The content of CYP3A seems to be affected by ethanol ingestion. Because ethanol is used widely, its potential interaction with CYP3A is of great interest. The effects of ethanol on CYP3A content and activity were assessed in different in vivo and in vitro models.

METHODS

Rats fed either the Lieber-DeCarli ethanol-containing diet or an ethanol and liquid diet via the intragastric tube feeding method were used. Additionally, HepG2 cell lines that constitutively and stably express human CYP3A4 were constructed to study ethanol interactions with CYP3A4.

RESULTS

In all models tested, ethanol induced CYP3A activity and content, as assessed by the metabolism of fentanyl, a sensitive and specific CYP3A substrate, and Western blot analysis, respectively. In the CYP3A4-expressing HepG2 cell line, incubation with ethanol caused a dose-dependent increase in CYP3A4 activity. Ethanol also increased messenger RNA levels of CYP3A4. In the HepG2-CYP3A4 line, incubation with cycloheximide caused a decrease in fentanyl metabolism secondary to a decrease in CYP3A4 levels; this decrease was prevented by coincubation of cycloheximide with ethanol.

CONCLUSIONS

Ethanol induced CYP3A activity and content both in vitro and in vivo. There may be multiple mechanisms of induction of CYP3A4 by ethanol, including stabilization of messenger RNA and protein. Ethanol-induced increases in both the protein level and activity of CYP3A4 may play a role that might be of pathophysiological or clinical significance.

Induction of CYP3A by ethanol in multiple in vitro and in vivo models

BACKGROUND

Cytochrome P-450 3A (CYP3A) is responsible for the metabolism of numerous therapeutic agents. The content of CYP3A seems to be affected by ethanol ingestion. Because ethanol is used widely, its potential interaction with CYP3A is of great interest. The effects of ethanol on CYP3A content and activity were assessed in different in vivo and in vitro models.

METHODS

Rats fed either the Lieber-DeCarli ethanol-containing diet or an ethanol and liquid diet via the intragastric tube feeding method were used. Additionally, HepG2 cell lines that constitutively and stably express human CYP3A4 were constructed to study ethanol interactions with CYP3A4.

RESULTS

In all models tested, ethanol induced CYP3A activity and content, as assessed by the metabolism of fentanyl, a sensitive and specific CYP3A substrate, and Western blot analysis, respectively. In the CYP3A4-expressing HepG2 cell line, incubation with ethanol caused a dose-dependent increase in CYP3A4 activity. Ethanol also increased messenger RNA levels of CYP3A4. In the HepG2-CYP3A4 line, incubation with cycloheximide caused a decrease in fentanyl metabolism secondary to a decrease in CYP3A4 levels; this decrease was prevented by coincubation of cycloheximide with ethanol.

CONCLUSIONS

Ethanol induced CYP3A activity and content both in vitro and in vivo. There may be multiple mechanisms of induction of CYP3A4 by ethanol, including stabilization of messenger RNA and protein. Ethanol-induced increases in both the protein level and activity of CYP3A4 may play a role that might be of pathophysiological or clinical significance.

Modulation of acetaminophen-induced hepatotoxicity by the xenobiotic receptor CAR

We have identified the xenobiotic receptor CAR (constitutive androstane receptor) as a key regulator of acetaminophen metabolism and hepatotoxicity. Known CAR activators as well as high doses of acetaminophen induced expression of three acetaminophen-metabolizing enzymes in wild-type but not in CAR null mice, and the CAR null mice were resistant to acetaminophen toxicity. Inhibition of CAR activity by administration of the inverse agonist ligand androstanol 1 hour after acetaminophen treatment blocked hepatotoxicity in wild type but not in CAR null mice. These results suggest an innovative therapeutic approach for treating the adverse effects of acetaminophen and potentially other hepatotoxic agents.

Should a lower treatment line be used when treating paracetamol poisoning in patients with chronic alcoholism?: a case for

A lower threshold for treatment of paracetamol (acetaminophen) poisoning has been advocated in chronic heavy users of alcohol, based originally on animal studies indicating that chronic alcohol ingestion increased hepatotoxicity. This was attributed to increased production of the toxic metabolite, N-acetyl-p-benzoquinoneimine, by cytochrome P450 (CYP)2E1 induction. The clinical evidence for increased risk is limited to four retrospective studies with potential for referral and reporting bias and conflicting results. No study has specifically addressed the issue of the treatment threshold for acute paracetamol overdose in chronic alcohol users. However, animal studies in multiple species have consistently shown a lower dose of paracetamol is required to produce hepatotoxicity after chronic alcohol use. The knowledge of potential mechanisms has expanded to include effects of other alcohols, such as isopentanol, induction of CYP enzymes other than CYP2E1 and glutathione depletion. There are no convincing reasons or data to suggest these findings do not apply to humans. However, further human toxicokinetic and clinical research is required to quantify the extent of the interaction. Arguments about treating overdoses should not be confused with those about whether there is an alcohol-paracetamol interaction at therapeutic doses. Halving the threshold dose/concentration for treatment is a conservative educated guess that has been widely adopted. In overdose, the potential benefits of treatment at this lower threshold clearly outweigh the minimal risks of acetylcysteine.

Acute versus chronic alcohol consumption in acetaminophen-induced hepatotoxicity

The aim of this study was to determine by multivariate analysis how alcohol and other factors affect the clinical course and outcome in patients with acetaminophen (paracetamol) poisoning. A total of 645 consecutive patients admitted from 1994 to 2000 with single-dose acetaminophen poisoning were studied, giving special attention to alcohol history, time between overdose and intravenous N-acetylcysteine (NAC) treatment ("time to NAC"), and other data available at the time of admittance. Up until 72 hours after ingestion, time to NAC was the single most important independent risk factor. With a time to NAC less than 12 hours, the mortality rate was 0.42% (95% CI, 0.05-2.7). When time to NAC exceeded 12, 24, and 48 hours, the mortality rate increased to 6.1%, 13%, and 19%, respectively. Chronic alcohol abuse was an independent risk factor of mortality (odds ratio [OR], 3.52; 95% CI, 1.78-6.97). Acute alcohol ingestion was an independent protective factor regarding mortality in alcoholic patients (OR, 0.08; 95% CI, 0.01-0.66) but not in nonalcoholic patients (OR, 0.21; 95% CI, 0.03-1.67). Patient age and quantity of acetaminophen were independent risk factors. In conclusion, time to NAC was confirmed as the major risk factor in acetaminophen-induced hepatotoxicity and mortality. Chronic alcohol abuse was an independent risk factor that could be counteracted by concomitant acute alcohol ingestion. We suggest that patients with chronic alcoholism and suspected acetaminophen poisoning due to an increased risk of developing hepatotoxicity should be treated with NAC regardless of risk estimation.

The paradoxical effect of acetaminophen on CYP3A4 activity and content in transfected HepG2 cells

HepG2 cell lines that constitutively and stably express human CYP3A4 were constructed in order to study enzyme interactions with CYP3A4 as the only P450 present. CYP3A4 activity and content were assessed by the metabolism of fentanyl, a CYP3A substrate, and Western blots. Northern blots were used to examine the effects of acetaminophen (APAP) on CYP3A4-mRNA. The HepG2 cell lines' CYP3A4 activity was stable over time. High concentrations of APAP inhibited CYP3A4 activity. At lower concentrations, APAP produced a dose-dependent increase in CYP3A4 activity and content. No increases in CYP3A4-mRNA were seen. Incubation with cycloheximide caused a decrease in fentanyl metabolism secondary to a decrease in P450 levels that was prevented by the coincubation with APAP. Additionally, human microsomal CYP3A4 was stabilized by APAP against cytosol-mediated degradation. In our models, APAP appears to increase CYP3A4 activity. This increase appears to be via substrate stabilization. This is the first report that APAP can increase CYP3A4 activity and content in transfected HepG2 cells.

Spatial glutathione and cysteine distribution and chemical modulation in the early organogenesis-stage rat conceptus in utero

Glutathione (GSH), cysteine, and other low-molecular-weight thiols (LMWT) play a vital role in the detoxication of xenobiotics and endogenous chemicals. Differential alterations of LMWT status in various cell types of the developing embryo may underlie cell-specific sensitivity or resistance to xenobiotics and contribute to embryotoxicity. This study describes the spatial and temporal distribution of LMWTs in rat conceptuses and alterations produced by the non-teratogenic GSH modulator, acetaminophen (APAP). Pregnant female rats were given 125, 250, or 500 mg/kg APAP (po) on gestational day 9. Conceptal LMWT was localized histochemically using mercury orange in cryosections, and GSH and cysteine concentrations were measured by HPLC analysis. Mercury orange histofluorescence revealed a non-uniform distribution of LMWT in untreated conceptal tissues, with strongest staining observed in the ectoplacental cone (EPC), visceral yolk sac (VYS), and embryonic heart. Less intense staining was observed in the neuroepithelium. Following treatment with APAP, tissue-associated LMWT decreased dramatically except in the EPC, while exocoelomic fluid LMWT, and LMWT within embryonic lumens, increased. Exposure to 250 mg/kg APAP decreased embryonic GSH after 6 and 24 h by 46% and 38%, respectively. Acetaminophen (500 mg/kg) decreased embryonic and VYS cysteine content by 54% and 83%, respectively, after 24 h. Acetaminophen alters the spatial distribution of LMWT in rat conceptuses, particularly with respect to cysteine. The mobilization of cysteine following chemical insult may influence the ability of conceptal cells to maintain normal GSH status due to reduced availability of cysteine for de novo GSH synthesis.

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.

Short-term treatment with alcohols causes hepatic steatosis and enhances acetaminophen hepatotoxicity in Cyp2e1(-/-) mice

CYP2E1 has been reported to have an essential role in alcohol-mediated increases in hepatic steatosis and acetaminophen hepatotoxicity. We found that pretreatment of Cyp2e1(-/-) mice with ethanol plus isopentanol, the predominant alcohols in alcoholic beverages, for 7 days resulted in micro- and macrovesicular steatosis in the livers of all mice, as well as a dramatic increase in acetaminophen hepatotoxicity. In Cyp2e1(-/-) mice administered up to 600 mg acetaminophen/kg alone and euthanized 7 h later, there was no increase in serum levels of ALT. In Cyp2e1(-/-) mice pretreated with ethanol and isopentanol, subsequent exposure to 400 or 600 mg acetaminophen/kg resulted in centrilobular necrosis in all mice with maximal elevation in serum levels of ALT. Acetaminophen-mediated liver damage was similar in males and females. Hepatic microsomal levels of APAP activation in untreated females were similar to those in males treated with the alcohols. However, the females, like the males, required pretreatment with the alcohols in order to increase APAP hepatotoxicity. These findings suggest that, in the Cyp2e1(-/-) mice, the alcohol-mediated increase in acetaminophen hepatotoxicity involves the contribution of other factors, in addition to induction of CYP(s) that activate acetaminophen. Alternatively, CYP-mediated activation of acetaminophen measured in vitro may not reflect the actual activity in vivo. Our findings that a 7-day treatment with ethanol and isopentanol causes extensive hepatic steatosis and increases acetaminophen hepatotoxicity in Cyp2e(-/-) mice indicate that CYP2E1 is not essential for either response.

Certain clinically-utilized antibiotics enhance the behavioural effects of cocaine

Abstract Cytochrome P-450s (CYPs) belonging to subfamilies 2B and 3A are the major CYPs involved in the N -demethylation of cocaine in the rat. However, the effects of inhibitors of these enzymes on the behavioural actions of cocaine are unknown. Hence, the effects of the CYP 3A inhibitors troleandomycin and erythromycin, and the CYP 2B (and 3A) inhibitor chloramphenicol, were examined on the locomotor activating effects of cocaine (20 mg/kg i.p.). Troleandomycin, chloramphenicol and erythromycin all potentiated the locomotor activating effects of cocaine, although the effect was only statistically significant for the first two drugs. Since variation exists in the human population with respect to the catalytic activity of CYP 3A isozymes, which are the principal cocaine N -demethylators in humans, inhibition of CYP 3A by troleandomycin in the rat may be useful as a model of the human cocaine "poor metabolizer" phenotype.

Paracetamol, alcohol and the liver

It is claimed that chronic alcoholics are at increased risk of paracetamol (acetaminophen) hepatotoxicity not only following overdosage but also with its therapeutic use. Increased susceptibility is supposed to be due to induction of liver microsomal enzymes by ethanol with increased formation of the toxic metabolite of paracetamol. However, the clinical evidence in support of these claims is anecdotal and the same liver damage after overdosage occurs in patients who are not chronic alcoholics. Many alcoholic patients reported to have liver damage after taking paracetamol with 'therapeutic intent' had clearly taken substantial overdoses. No proper clinical studies have been carried out to investigate the alleged paracetamol-alcohol interaction and acute liver damage has never been produced by therapeutic doses of paracetamol given as a challenge to a chronic alcoholic. The paracetamol-alcohol interaction is complex; acute and chronic ethanol have opposite effects. In animals, chronic ethanol causes induction of hepatic microsomal enzymes and increases paracetamol hepatotoxicity as expected (ethanol primarily induces CYP2E1 and this isoform is important in the oxidative metabolism of paracetamol). However, in man, chronic alcohol ingestion causes only modest (about twofold) and short-lived induction of CYP2E1, and there is no corresponding increase (as claimed) in the toxic metabolic activation of paracetamol. The paracetamol-ethanol interaction is not specific for any one isoform of cytochrome P450, and it seems that isoenzymes other than CYP2E1 are primarily responsible for the oxidative metabolism of paracetamol in man. Acute ethanol inhibits the microsomal oxidation of paracetamol both in animals and man. This protects against liver damage in animals and there is evidence that it also does so in man. The protective effect disappears when ethanol is eliminated and the relative timing of ethanol and paracetamol intake is critical. In many of the reports where it is alleged that paracetamol hepatotoxicity was enhanced in chronic alcoholics, the reverse should have been the case because alcohol was actually taken at the same time as the paracetamol. Chronic alcoholics are likely to be most vulnerable to the toxic effects of paracetamol during the first few days of withdrawal but maximum therapeutic doses given at this time have no adverse effect on liver function tests. Although the possibility remains that chronic consumption of alcohol does increase the risk of paracetamol hepatotoxicity in man (perhaps by impairing glutathione synthesis), there is insufficient evidence to support the alleged major toxic interaction. It is astonishing that clinicians and others have unquestion-ingly accepted this supposed interaction in man for so long with such scant regard for scientific objectivity.

Acetaminophen hepatotoxicity precipitated by short-term treatment of rats with ethanol and isopentanol: protection by triacetyloleandomycin

Ethanol and isopentanol are the predominant alcohols in alcoholic beverages. We have reported previously that pretreatment of rats with a liquid diet containing 6.3% ethanol plus 0.5% isopentanol for 7 days results in a synergistic increase in acetaminophen hepatotoxicity, compared with rats treated with either alcohol alone. Here, we investigated the role of CYP3A in acetaminophen hepatotoxicity associated with the combined alcohol treatment. Triacetyloleandomycin, a specific inhibitor of CYP3A, protected rats pretreated with ethanol along with isopentanol from acetaminophen hepatotoxicity. At both 0.25 and 0.5 g acetaminophen/kg, triacetyloleandomycin partially prevented elevations in serum levels of alanine aminotransferase. At 0.25 g acetaminophen/kg, triacetyloleandomycin completely protected 6 of 8 rats from histologically observed liver damage, and partially protected the remaining 2 rats. At 0.5 g acetaminophen/kg, triacetyloleandomycin decreased histologically observed liver damage in 7 of 15 rats. In rats pretreated with ethanol plus isopentanol, CYP3A, measured immunohistochemically, was decreased by acetaminophen treatment. This effect was prevented by triacetyloleandomycin. These results suggest that CYP3A has a major role in acetaminophen hepatotoxicity in animals administered the combined alcohol treatment. We also found that exposure to ethanol along with 0.1% isopentanol for only 3 days resulted in maximal increases in acetaminophen hepatotoxicity by the combined alcohol treatment, suggesting that short-term consumption of alcoholic beverages rich in isopentanol may be a risk for developing liver damage from acetaminophen.

Phenytoin as a possible cause of acetaminophen hepatotoxicity: case report and review of the literature

A 55-year-old woman was hospitalized for treatment of community-acquired pneumonia. Unexplained, moderate elevations in hepatic transaminase and enzyme levels prompted review of her drug regimen. She had taken acetaminophen 1,300-6,200 mg/day during the hospitalization. She also received phenytoin for posttraumatic seizures. Acetaminophen was discontinued, and the patient's liver chemistries returned to normal within 2 weeks of discharge. Acetaminophen is metabolized in part by cytochrome P450 (CYP) 2E1, and inducers of CYP2E1 are known to predispose patients to acetaminophen-related hepatotoxicity. Phenytoin induces CYP2C and CYP3A4 isoforms, but not CYP2E1. The literature suggests, however, that CYP3A4 may participate in acetaminophen metabolism to a greater extent than previously realized, and induction of this isoform may predispose patients to acetaminophen-induced hepatotoxicity.

CYP2E1 and CYP3A1/2 gene expression is not associated with the ursodeoxycholate effect on ethanol-induced lipoperoxidation

Ethanol is a well-known hepatotoxicant inducing steatosis and membrane lipoperoxidation. The aim of the present study was to investigate in rats, whether the protective effect of UDC on ethanol-induced lipid peroxidation may be related with CYP2E1 and CYP3A1/2 gene expression. We showed that UDC treatment in ethanol-fed rats induced a significant decrease in liver triglyceride concentration which was closely correlated with a reduction in malondialdehyde and hydroxyalkenal levels. In chronically ethanol-fed rats, CYP2E1 and CYP3A1/2 gene expressions were increased by a post-transcriptional mechanism. These inductions, mainly of CYP2E1, could take part in alcohol-induced hepatic lipoperoxidation. UDC modified neither the specific activity, nor the protein level, nor the mRNA level of CYP2E1 when compared with control. UDC supplementation to alcohol diet did not prevent the increase in CYP2E1 expression of ethanol-fed rats. Furthermore, CYP3A1/2 protein levels were similarly increased by ethanol and ethanol plus UDC treatment. Therefore, UDC protective effect against ethanol-induced lipoperoxidation was not associated with a modification of CYP2E1 and CYP3A1/2 expression.

Optimization of treatment conditions for studying the anticancer effects of retinoids using pancreatic adenocarcinoma as a model

Retinoids are natural differentiation-inducing compounds that are promising as anticancer agents. Cancer cell lines are valuable in the investigation of the potential of retinoids for the treatment of specific cancers. However, using different treatment conditions but the same cell lines, investigators have produced markedly contradictory results for the effectiveness of retinoids. The present study examined different factors in the treatment conditions that may have masked or interfered with the effects of retinoids and, thereby, resulted in this conflict. Our studies revealed that the effects of retinoids on cancer cell proliferation were influenced by serum, the choice of vehicle (DMSO vs ethanol) and its concentration, phenol red, the degree of cellular confluence, and the method of assessing proliferation (cell number or [3H]thymidine uptake vs the MTT assay). Optimized conditions were the use of serum-free, ethanol-free, and phenol red-free media, investigating cells in the log phase of growth, using </=0.01% DMSO as the vehicle, and monitoring proliferation by cell number or [3H]thymidine incorporation into DNA measured after TCA precipitation. Using these conditions, retinoids were found to exhibit potent antiproliferative effects in pancreatic cancer cells with a variety of degrees of differentiation, even in cell lines previously documented as being retinoid resistant. Retinoids also induced morphological changes and cellular death that may indicate terminal differentiation and apoptosis.

Autoantibodies against cytochromes P-4502E1 and P-4503A in alcoholics

Autoantibodies against soluble liver enzymes have been reported among alcoholics, but the targets of self-reactivity toward membrane proteins of the liver have not been characterized. Previously, among alcoholics, we found antibodies against ethanol-derived radical protein adducts that are dependent on cytochrome P-4502E1 (CYP2E1) for their formation. To further investigate autoantibodies against cytochrome P-450s during alcohol abuse, sera of rats chronically treated with ethanol in the total enteral nutrition model and sera from alcoholics with or without alcohol liver disease and from control subjects were analyzed by enzyme-linked immunosorbent assay and Western blotting for the presence of IgG against rat and human CYP2E1, rat CYP3A1, and human CYP3A4. A time-dependent appearance of IgG against rat CYP3A1 and CYP2E1 was evident during chronic ethanol feeding of rats. Anti-CYP2E1 reactivity showed positive correlation with the levels of hepatic CYP2E1 and was inhibited by the CYP2E1 transcriptional inhibitor chlormethiazole. Screening of the human sera by enzyme-linked immunosorbent assay revealed reactivity against CYP3A4 and CYP2E1 in about 20 to 30% and 10 to 20% of the alcoholic sera, respectively. No difference were noted between sera from alcoholics with or without hepatitis C virus infection, and only very little reactivity was seen in sera from control subjects. Western blotting analysis revealed anti-human CYP2E1 reactivity in 8 of 85 alcoholic sera and 3 of 58 control sera, whereas anti-CYP3A4 reactivity was detected in 18 of 85 alcoholic sera and 4 of 58 control sera, which were different from the sera reactive with CYP2E1. Immunoblot reactivity of CYP3A4-positive alcoholic sera was found against glutathione-S-transferase fusion proteins containing truncated forms of CYP3A4, and such sera were also able to immunoprecipitate in vitro translated CYP3A4. Seven of eight sera showed reactivity toward domains C-terminal of position Ser281, and 1 of 8 sera recognized autoepitopes within the region Thr207-Ser281. These findings indicate that alcoholics develop autoantibodies against CYP2E1 and CYP3A4 that the CYP3A4 C-terminal domain is a target for the autoantibody reactions among a subset of alcoholics. The novel finding of CYP3A4 autoantibodies and their significant expression among alcoholics warrants further investigation. Attention should be given to immune toxicity associated with CYP3A4 autoantibodies and cases of alcohol abuse that are accompanied by exposure to drugs and substances that are CYP3A substrates.

Acarbose alone or in combination with ethanol potentiates the hepatotoxicity of carbon tetrachloride and acetaminophen in rats

Acarbose reduces the absorption of monosaccharides derived from dietary carbohydrates, which play an important role in the metabolism and toxicity of some chemical compounds. We studied the effects of acarbose on the hepatotoxicity of carbon tetrachloride (CCl4) and acetaminophen (AP) in rats, both of which exert their toxic effects through bioactivation associated with cytochrome P450 2E1 (CYP2E1). Male Sprague-Dawley rats were kept on a daily ration (20 g) of powdered chow diet containing 0, 20, 40, or 80 mg/100 g of acarbose, with drinking water containing 0% or 10% of ethanol (vol/vol). Three weeks later, the rats were either killed for an in vitro metabolism study or challenged with 0.50 g/kg CCl4 orally or 0. 75 g/kg AP intraperitoneally. The ethanol increased the hepatic microsomal CYP2E1 level and the rate of dimethylnitrosamine (DMN) demethylation. The 40- or 80-mg/100 g acarbose diet, which alone increased the CYP2E1 level and the rate of DMN demethylation, augmented the enzyme induction by ethanol. The 40- or 80-mg/100 g acarbose diet alone potentiated CCl4 and AP hepatotoxicity, as evidenced by significantly increased levels of both alanine transaminase (ALT) and aspartate transaminase (AST) in the plasma of rats pretreated with acarbose. Ethanol alone also potentiated the toxicity of both chemicals. When the 40- or 80-mg/100 g acarbose diet was combined with ethanol, the ethanol-induced potentiation of CCl4 and AP hepatotoxicity was augmented. Our study demonstrated that high doses of acarbose, alone or in combination with ethanol, can potentiate CCl4 and AP hepatotoxicity in rats by inducing hepatic CYP2E1.