Induction of rat hepatic P450IIE1 (CYP 2E1) by pyridine: evidence for a role of protein synthesis in the absence of transcriptional activation

Alcoholic Liver Disease: Alcohol Metabolism, Cascade of Molecular Mechanisms, Cellular Targets, and Clinical Aspects

Alcoholic liver disease is the result of cascade events, which clinically first lead to alcoholic fatty liver, and then mostly via alcoholic steatohepatitis or alcoholic hepatitis potentially to cirrhosis and hepatocellular carcinoma. Pathogenetic events are linked to the metabolism of ethanol and acetaldehyde as its first oxidation product generated via hepatic alcohol dehydrogenase (ADH) and the microsomal ethanol-oxidizing system (MEOS), which depends on cytochrome P450 2E1 (CYP 2E1), and is inducible by chronic alcohol use. MEOS induction accelerates the metabolism of ethanol to acetaldehyde that facilitates organ injury including the liver, and it produces via CYP 2E1 many reactive oxygen species (ROS) such as ethoxy radical, hydroxyethyl radical, acetyl radical, singlet radical, superoxide radical, hydrogen peroxide, hydroxyl radical, alkoxyl radical, and peroxyl radical. These attack hepatocytes, Kupffer cells, stellate cells, and liver sinusoidal endothelial cells, and their signaling mediators such as interleukins, interferons, and growth factors, help to initiate liver injury including fibrosis and cirrhosis in susceptible individuals with specific risk factors. Through CYP 2E1-dependent ROS, more evidence is emerging that alcohol generates lipid peroxides and modifies the intestinal microbiome, thereby stimulating actions of endotoxins produced by intestinal bacteria; lipid peroxides and endotoxins are potential causes that are involved in alcoholic liver injury. Alcohol modifies SIRT1 (Sirtuin-1; derived from Silent mating type Information Regulation) and SIRT2, and most importantly, the innate and adapted immune systems, which may explain the individual differences of injury susceptibility. Metabolic pathways are also influenced by circadian rhythms, specific conditions known from living organisms including plants. Open for discussion is a 5-hit working hypothesis, attempting to define key elements involved in injury progression. In essence, although abundant biochemical mechanisms are proposed for the initiation and perpetuation of liver injury, patients with an alcohol problem benefit from permanent alcohol abstinence alone.

Insights into insulin-mediated regulation of CYP2E1: miR-132/-212 targeting of CYP2E1 and role of phosphatidylinositol 3-kinase, Akt (protein kinase B), mammalian target of rapamycin signaling in regulating miR-132/-212 and miR-122/-181a expression in primary cultured rat hepatocytes

Several microRNAs (miRNAs) were selected for characterization of their response to insulin signaling based on in silico predictions of targeting CYP2E1 mRNA and previous reports implicating their role in hepatic metabolism and disease. CYP2E1 expression decreases with increasing insulin concentration and has been shown to be regulated by the phosphatidylinositol 3-kinase (PI3-K)/protein kinase B (Akt)/mammalian target of rapamycin (mTOR) signaling pathway. In primary cultured rat hepatocytes, insulin at 0.1, 1.0, and 10 nM elevated miRNA-132 and -212 expression ∼2- and 1.8-fold, respectively, whereas expression of miRNA-181a and -122 increased ∼1.6- and 1.4-fold, respectively. In contrast, insulin failed to alter significantly the expression of miRNA let-7a. Mechanistic studies using inhibitors of PI3-K, Akt, and mTOR were used to examine the role of the insulin signaling pathway on miR expression and resulted in significant suppression of the insulin-mediated elevation of miR-132, miR-212, and miR-122 levels, with a lesser effect observed for miR-181a. Targeting of the rat CYP2E1 3'-untranslated region (UTR) by miR-132 and -212 was demonstrated with an in vitro luciferase reporter assay. These data show that insulin, which regulates CYP2E1 through the PI3-K, Akt, mTOR signaling pathway, also regulates the expression of miRs that target the 3'-UTR of CYP 2E1 mRNA and are involved in the regulation of hepatic metabolism and disease.

Genetic polymorphism in CYP2E1: Population distribution of CYP2E1 activity

Cytochrome P-450 2E1 (CYP2E1) is a key enzyme in the metabolic activation of a variety of toxicants including nitrosamines, benzene, vinyl chloride, and halogenated solvents such as trichloroethylene. CYP2E1 is also one of the enzymes that metabolizes ethanol to acetaldehyde, and is induced by recent ethanol ingestion. There is evidence that interindividual variability in the expression and functional activity of this cytochrome (CYP) may be considerable. Genetic polymorphisms in CYP2E1 were identified and linked to altered susceptibility to hepatic cirrhosis induced by ethanol and esophageal and other cancers in some epidemiological studies. Therefore, it is important to evaluate how such polymorphisms affect CYP2E1 function and whether it is possible to construct a population distribution of CYP2E1 activity based upon the known effects of these polymorphisms and their frequency in the population. This analysis is part of the genetic polymorphism database project described in the lead article in this series and followed the approach described in that article (Ginsberg et al., 2009, this issue). Review of the literature found that there are a variety of CYP2E1 variant alleles but the functional significance of these variants is still unclear. Some, but not all, studies suggest that several upstream 5' flanking mutations affect gene expression and response to inducers such as ethanol or obesity. None of the coding-region variants consistently affects enzyme function. Part of the reason for conflicting evidence regarding genotype effect on phenotype may be due to the wide variety of exposures such as ethanol or dietary factors and physiological factors including body weight or diabetes that modulate CYP2E1 expression. In conclusion, evidence is too limited to support the development of a population distribution of CYP2E1 enzyme activity based upon genotypes. Health risk assessments may best rely upon data reporting interindividual variability in CYP2E1 function for input into physiologically based pharmacokinetic (PBPK) models involving CYP2E1 substrates.

Protective effects of puerarin on carbon tetrachloride-induced hepatotoxicity

Puerarin, the main isoflavone glycoside found in the root of Pueraria lobata, has been used for various medicinal purposes in traditional Chinese medicine for thousands of years. The purpose of this study was to investigate the protective effects of puerarin against hepatotoxicity induced by carbon tetrachloride (CCl4) and the mechanism of its hepatoprotective effect. In mice, pretreatment with puerarin prior to the administration of CCl4 significantly prevented the increased serum enzymatic activity of alanine aspartate aminotransferase and hepatic malondialdehyde formation in a dose-dependent manner. In addition, pretreatment with puerarin significantly prevented both the depletion of reduced glutathione (GSH) content and the decrease in glutathione S-transferase (GST) activity in the liver of CCl4-intoxicated mice. Hepatic GSH levels and GST activity were increased by treatment with puerarin alone. CCl4-induced hepatotoxicity was also prevented, as indicated by liver histopathology. The effects of puerarin on cytochrome P450 (CYP) 2E1, the major isozyme involved in CCl4 bioactivation, were also investigated. Treatment of the mice with puerarin resulted in a significant decrease in the CYP2E1-dependent aniline hydroxylation in a dose-dependent manner. Consistent with these observations, the CYP2E1 protein levels were also lowered. Puerarin exhibited anti-oxidant effects on FeCl2-ascorbate induced lipid peroxidation in mouse liver homogenates, and on superoxide radical scavenging activity. These results suggest that the protective effects of puerarin against the CCl4-induced hepatotoxicity possibly involve mechanisms related to its ability to block CYP-mediated CCl4 bioactivation, induction of GST activity and free radical scavenging effects.

Expression and characterization of cytochrome P450 2X1 in channel catfish (Ictalurus punctatus)

Previous studies in channel catfish identified a novel cDNA encoding the cytochrome P450 isoform, CYP2X1. To characterize the substrate specificity of CYP2X1, the 57 kDa protein was expressed in Sf9 cells. Microsomes from Sf9 cells transfected with CYP2X1 demonstrated a maximum carbon monoxide-reduced difference spectrum at 450 nm and catalyzed aminopyrine and benzphetamine demethylase activity with catalytic efficiency (Vmax/Km) values of 0.82 pmol/nmol P450/min and 4.39 pmol/nmol P450/min, respectively. However, enzymatic activity was not observed following incubation with p-nitrophenol, benzyloxyresorufin or pentoxyresorufin. Expression of CYP2X1 transcription was significantly elevated in the gills and liver relative to that detected in brain, kidney and heart. In the brain, liver and heart, intraperitoneal injections with clofibric acid, ethanol, pyridine and rifampin failed to alter expression of CYP2X1 mRNA. In kidney, pyridine significantly suppressed the expression of CYP2X1 transcription (p < or = 0.05). These results indicate CYP2X1 displays minimal catalytic activities consistent with other piscine CYP2 isoforms, and unique tissue expression and regulation patterns in juvenile channel catfish.

Induction and recovery time course of rat brain CYP2E1 after nicotine treatment

CYP2E1, the primary ethanol-metabolizing cytochrome P450, metabolizes endogenous substrates (e.g., arachidonic acid) and drugs (e.g., acetaminophen, chlorzoxazone) and bioactivates procarcinogens (e.g., tobacco-specific nitrosamines) and toxins (e.g., carbon tetrachloride). Nicotine from tobacco smoke may contribute to the enhanced hepatic CYP2E1 activity in smokers. We have previously shown that chronic nicotine treatment can increase CYP2E1 in rat liver and brain. In this study, induction of brain CYP2E1 was assessed after a single acute or a 7-day chronic treatment with saline or nicotine (1 mg/kg s.c.), with sacrifice performed at various times after the last injection. Chronic 7-day nicotine treatment showed the highest levels of CYP2E1 12 h after the last injection in frontal cortex (1.4-fold, p < 0.05) versus 8 h in hippocampus (1.8-fold, p < 0.01) and cerebellum (1.4-fold, p < 0.05), returning to basal levels by 24 h. In contrast, acute nicotine treatment did not induce CYP2E1 in frontal cortex and hippocampus but increased CYP2E1 in cerebellum 8 h after treatment (1.6-fold, p < 0.01). Brain CYP2E1 mRNA levels did not increase after chronic nicotine treatment, suggesting nontranscriptional regulation. Thus, humans exposed to nicotine may have altered CYP2E1-mediated metabolism of centrally acting drugs and toxins as well as altered toxicity because of oxidative stress caused by CYP2E1. Those affected may include current and passive smokers and people that may be treated with nicotine such as smokers and, potentially, patients with Alzheimer's, Parkinson's disease, or ulcerative colitis.

The discovery of the microsomal ethanol oxidizing system and its physiologic and pathologic role

Oxidation of ethanol via alcohol dehydrogenase (ADH) explains various metabolic effects of ethanol but does not account for the tolerance. This fact, as well as the discovery of the proliferation of the smooth endoplasmic reticulum (SER) after chronic alcohol consumption, suggested the existence of an additional pathway which was then described by Lieber and DeCarli, namely the microsomal ethanol oxidizing system (MEOS), involving cytochrome P450. The existence of this system was initially challenged but the effect of ethanol on liver microsomes was confirmed by Remmer and his group. After chronic ethanol consumption, the activity of the MEOS increases, with an associated rise in cytochrome P450, especially CYP2E1, most conclusively shown in alcohol dehydrogenase negative deer mice. There is also cross-induction of the metabolism of other drugs, resulting in drug tolerance. Furthermore, the conversion of hepatotoxic agents to toxic metabolites increases, which explains the enhanced susceptibility of alcoholics to the adverse effects of various xenobiotics, including industrial solvents. CYP2E1 also activates some commonly used drugs (such as acetaminophen) to their toxic metabolites, and promotes carcinogenesis. In addition, catabolism of retinol is accelerated resulting in its depletion. Contrasting with the stimulating effects of chronic consumption, acute ethanol intake inhibits the metabolism of other drugs. Moreover, metabolism by CYP2E1 results in a significant release of free radicals which, in turn, diminishes reduced glutathione (GSH) and other defense systems against oxidative stress which plays a major pathogenic role in alcoholic liver disease. CYP1A2 and CYP3A4, two other perivenular P450s, also sustain the metabolism of ethanol, thereby contributing to MEOS activity and possibly liver injury. CYP2E1 has also a physiologic role which comprises gluconeogenesis from ketones, oxidation of fatty acids, and detoxification of xenobiotics other than ethanol. Excess of these physiological substrates (such as seen in obesity and diabetes) also leads to CYP2E1 induction and nonalcoholic fatty liver disease (NAFLD), which includes nonalcoholic fatty liver and nonalcoholic steatohepatitis (NASH), with pathological lesions similar to those observed in alcoholic steatohepatitis. Increases of CYP2E1 and its mRNA prevail in the perivenular zone, the area of maximal liver damage. CYP2E1 up-regulation was also demonstrated in obese patients as well as in rat models of obesity and NASH. Furthermore, NASH is increasingly recognized as a precursor to more severe liver disease, sometimes evolving into "cryptogenic" cirrhosis. The prevalence of NAFLD averages 20% and that of NASH 2% to 3% in the general population, making these conditions the most common liver diseases in the United States. Considering the pathogenic role that up-regulation of CYP2E1 also plays in alcoholic liver disease (vide supra), it is apparent that a major therapeutic challenge is now to find a way to control this toxic process. CYP2E1 inhibitors oppose alcohol-induced liver damage, but heretofore available compounds are too toxic for clinical use. Recently, however, polyenylphosphatidylcholine (PPC), an innocuous mixture of polyunsaturated phosphatidylcholines extracted from soybeans (and its active component dilinoleoylphosphatidylcholine), were discovered to decrease CYP2E1 activity. PPC also opposes hepatic oxidative stress and fibrosis. It is now being tested clinically.

Protective Effect of Acteoside on Carbon Tetrachloride-Induced Hepatotoxicity

This study investigated the protective effects of acteoside, a phenylethanoid glycoside, on the carbon tetrachloride-induced hepatotoxicity as well as the possible mechanisms involved in this protection in mice. Pretreatment with acteoside prior to the administration of carbon tetrachloride significantly prevented the increased serum enzymatic activities of alanine and aspartate aminotransferase in a dose-dependent manner. In addition, pretreatment with acteoside significantly prevented the increase in hepatic malondialdehyde formation and the depletion of the reduced glutathione content in the liver of carbon tetrachloride-intoxicated mice. Carbon tetrachloride-induced hepatotoxicity was also essentially prevented, as indicated by a liver histopathologic study. The effects of acteoside on cytochrome P450 (P450) 2E1, the major isozyme involved in carbon tetrachloride bioactivation were also investigated. Treatment of the mice with acteoside resulted in a significant decrease in the P450 2E1-dependent pnitrophenol and aniline hydroxylation in a dose-dependent manner. Consistent with these observations, the P450 2El protein levels were also lower. Acteoside exhibited anti-oxidant effects on FeCl2-ascorbate induced lipid peroxidation in a mouse liver homogenate, and on superoxide radical scavenging activity. These results suggest that the protective effects of acteoside against the carbon tetrachloride-induced hepatotoxicity possibly involve mechanisms related to its ability to block the P450-mediated carbon tetrachloride bioactivation and free radical scavenging effects.

Rat hepatic CYP2E1 is induced by very low nicotine doses: an investigation of induction, time course, dose response, and mechanism

CYP2E1 is an ethanol- and drug-metabolizing enzyme that can also activate procarcinogens and hepatotoxicants and generate reactive oxygen species; it has been implicated in the pathogenesis of liver diseases and cancer. Cigarette smoke increases CYP2E1 activity in rodents and in humans and we have shown that nicotine (0.1-1.0 mg/kg s.c. x 7 days) increases CYP2E1 protein and activity in the rat liver. In the current study, we have shown that the induction peaks at 4 h postnicotine (1 mg/kg s.c. x 7 days) treatment and recovers within 24 h. No induction was observed after a single injection, and 18 days of treatment did not increase the levels beyond that found at 7 days. We found that CYP2E1 is induced by very low doses of chronic (x 7 days) nicotine with an ED50 value of 0.01 mg/kg s.c.; 0.01 mg/kg in a rat model results in peak cotinine levels (nicotine metabolite) similar to those found in people exposed to environmental tobacco smoke (passive smokers; 2-7 ng/ml). Previously, we have shown no change in CYP2E1 mRNA, and our current mechanistic study indicates that nicotine does not regulate CYP2E1 expression by protein stabilization. We postulated that a nicotine metabolite could be causing the induction but found that cotinine (1 mg/kg x 7 days) did not increase CYP2E1. Our findings indicate that nicotine increases CYP2E1 at very low doses and may enhance CYP2E1-related toxicity in smokers, passive smokers, and people treated with nicotine (e.g., smokers, patients with Alzheimer's disease, ulcerative colitis or Parkinson's disease).

Glucose inhibition of the induction of CYP2E1 mRNA expression by ethanol in FGC-4 cells

1. Rats fed intragastrically with ethanol-containing diets made with low levels of carbohydrates have greater CYP2E1 induction than rats fed similar diets made with high carbohydrate levels. 2. FGC-4 rat hepatoma cells were used to test the hypothesis that carbohydrates could down-regulate ethanol-induced CYP2E1 induction. 3. FGC-4 cells grown in a glucose-free media and treated with 1-100 mM ethanol for 24 h exhibited a dose-dependent increase (p < 0.05) in CYP2E1, with maximum mRNA steady-state (3.8-fold) or protein (3.1-fold) levels measured at 30 or 100 mM ethanol, respectively. 4. In cells treated with 30 mM ethanol, a glucose concentration-dependent inhibition (p < 0.05) of CYP2E1 mRNA was observed between 2.5 and 10 mM glucose. 5. Induction by 30 mM ethanol of CYP2E1 protein was reduced in cells co-treated with 1 mM or greater glucose concentration and complete inhibition was measured with 5 mM glucose co-treatment. 6. These data demonstrate that under culture conditions of extremely low carbohydrate concentrations: (1) ethanol treatment of FGC-4 cells results in elevated steady-state levels of CYP2E1 mRNA and protein; and (2) glucose inhibits this increase. 7. It is concluded that glucose can negatively regulate CYP2E1 expression and could at least partially explain the greater induction of hepatic CYP2E1 in rats fed low carbohydrate ethanol-containing diets compared with high carbohydrate diets at the same ethanol level.

Oxidative damage to the hepatocellular proteins after chronic ethanol intake in the rat

BACKGROUND

Protein carbonyl content, a measure of oxidative damage to hepatocellular proteins, and the activities of some thiol-containing proteins were assayed in the liver and plasma, as thiol-containing protein, appear to be targets for free radicals. These may be important in the mechanism of ethanol-induced liver injury.

METHODS

Tap water containing ethanol at the concentration of 25% (v/v) and phenobarbital (500 mg/l) was the only source of drinking water for the experimental rats for 24 months. Another group of rats were administered 25% (v/v) ethanol alone in drinking water for 24 months. Control rats were administered either phenobarbital alone in drinking water or tap water for 24 months. At the end of 24 months, the rats were sacrificed. The protein carbonyl content, activities of glutamine synthase and biotinidase-sulfhydryl group containing enzymes were assayed in the liver along with alkaline protease, an enzyme that degrades oxidized proteins. The total thiol, albumin and the activity of biotinidase were measured in the plasma.

RESULTS

The protein carbonyl content of the liver was increased in the ethanol/phenobarbital-treated rats as well as in the ethanol-treated rats as compared with the controls. The activities of glutamine synthase and biotinidase were decreased significantly in the livers of ethanol/phenobarbital-treated rats as well as the ethanol-treated rats as compared with the controls. The activity of alkaline protease was increased significantly in both the ethanol-treated groups. In the plasma of ethanol/phenobarbital-treated rats as well as the ethanol-treated rats total thiol, albumin and the activity of biotinidase were decreased significantly as compared with the controls. The ethanol/phenobarbital-treated rats as well as the ethanol-treated rats developed fatty liver.

CONCLUSIONS

Damage to proteins occurs upon chronic ethanol intake in the rat, and it may play a role in the pathogenesis of alcohol-induced fatty liver.

The Human Cytochrome P450 1A1 mRNA Is Rapidly Degraded In HepG2 Cells

The cytochromes P450 are a superfamily of enzymes that can carry out a wide range of oxidative reactions. While the transcriptional control of the cytochrome P450 genes has been relatively well-studied, posttranscriptional regulatory mechanisms that contribute to the regulation of P450s are much less well understood. We followed the decay of CYP1A1, CYP1A2, and CYP1B1 mRNAs after induction by the AH receptor ligand 2,3,7,8,-tetrachlorodibenzo-p-dioxin. CYP1A2 and CYP1B1 mRNAs were long-lived in this cell line (to > 24 h). In contrast, the CYP1A1 mRNA decays remarkably quickly. To determine if this rapid decay was unique to CYP1A1, we assessed the decay of selected human P450 and liver-specific mRNAs in HepG2 cells as a comparison. We analyzed albumin, phosphofructokinase, and GAPDH mRNAs and found that they were long-lived, with half-lives >24 h. We show that CYP2E1 mRNA can be detected in HepG2 cells by RT-PCR and that this mRNA also has a basal half-life of >24 h. Thus the CYP1A1 mRNA with its half-life of 2.4 h was one of the shortest-lived mRNA studied and is the most unstable of the cytochrome P450 mRNAs we have tested. The rapid decay of CYP1A1 mRNA is associated with a rapid loss in poly(A) tail length, suggesting that deadenylation is the first step in the decay pathway. The short half-life appears to be conserved across species, which suggests that this characteristic of the CYP1A1 mRNA is important for its function.

Promoter function and the role of cytokines in the transcriptional regulation of rabbit CYP2E1 and CYP2E2

Rabbit CYP2E1 and CYP2E2 show considerable similarity in the 5' flanking region, but a 32-base-pair element (32-BPE) that is repeated in 2E1 is present only as a single inexact copy in 2E2. In the present investigation, footprinting disclosed two specific binding sites for liver nuclear proteins, and the DNase I sensitivity profiles of the two genes were found to be different. Several positive and negative regulatory elements were identified by transfection with a series of constructs of upstream CYP2E sequences fused to the luciferase gene. Both genes have an HNF-1 consensus motif with one nucleotide mismatch, which affects binding affinity and promoter activity. Investigation of DNA-protein interactions revealed that Sp1 and NFkappaB bind exclusively to the 32-BPE of 2E1 and 2E2, respectively, suggesting a possible regulatory role for the 32-BPE. Interleukin-1alpha (IL-1alpha) gave rise to a 2.5-fold increase in the promoter activity of 2E1 in HepG2 cells, and the IL-1alpha-mediated induction of reporter gene expression was almost completely prevented when the 32-BPE was deleted. Increased DNA binding and Sp1 protein content as a result of IL-1alpha treatment, as well as cotransfection experiments with pPacSp1, suggest that Sp1 is a transcription activator for the induction of 2E1 by IL-1alpha in HepG2 cells.

The alcohol-inducible form of cytochrome P450 (CYP 2E1): role in toxicology and regulation of expression

Cytochrome P450 (CYP) 2E1 catalyzes the metabolism of a wide variety of therapeutic agents, procarcinogens, and low molecular weight solvents. CYP2E1-catalyzed metabolism may cause toxicity or DNA damage through the production of toxic metabolites, oxygen radicals, and lipid peroxidation. CYP2E1 also plays a role in the metabolism of endogenous compounds including fatty acids and ketone bodies. The regulation of CYP2E1 expression is complex, and involves transcriptional, post-transcriptional, translational, and post-translational mechanisms. CYP2E1 is transcriptionally activated in the first few hours after birth. Xenobiotic inducers elevate CYP2E1 protein levels through both increased translational efficiency and stabilization of the protein from degradation, which appears to occur primarily through ubiquitination and proteasomal degradation. CYP2E1 mRNA and protein levels are altered in response to pathophysiologic conditions by hormones including insulin, glucagon, growth hormone, and leptin, and growth factors including epidermal growth factor and hepatocyte growth factor, providing evidence that CYP2E1 expression is under tight homeostatic control.

Cell biology of cytochrome P-450 in the liver

Cytochromes P-450 (P-450) are members of a multigene superfamily of hemoproteins consisting the microsomal monooxygenase system with NADPH P-450 reductase (reductase) and/or reducing equivalents. Expression of many P-450 isoforms in hepatocytes is shown to be regulated at the level of transcription through interaction between cis-acting elements in the genes and DNA-binding (transacting) factors. Some isoforms of the CYP1A, 2B, 2E, and 3A subfamilies are regulated at the posttranscriptional level. For the topology of P-450 and reductase molecules in ER membrane of hepatocytes, models from stopped flow analysis and electron spin resonance are proposed. The densities of total P-450 and reductase molecules are revealed to be high enough to support the cluster model, suggesting that about ten P-450 molecules form an aggregate and surround one reductase molecule, and therefore the two enzymes form large micelles. ER proliferation after PB administration, which had been correlated with increase in P-450 level, is shown to be probably independent of the increase in P-450 level. There are considerable discrepancies among results reported on sublobular expression of various P-450 isoforms. Causes of the discrepancies are likely to be differences in experimental conditions of histochemical detection carried out and/or in species, strain, and/or sex.

Hormonal regulation of microsomal cytochrome P4502E1 and P450 reductase in rat liver and kidney

1. The relative roles of pituitary hormones (especially growth hormone) and testicular hormones (especially testosterone) in the regulation of renal and hepatic CYP2E1 and cytochrome P450 reductase have been studied in the male rat. 2. Depletion of pituitary hormones by hypophysectomy (Hx) resulted in 12-14-fold increases in renal CYP2E1 (p < or = 0.05) and a 40% drop in NADPH-dependent cytochrome c reductase activity (p < or = 0.05) compared with 6-fold increases in CYP2E1 (p < or = 0.05) and a 60% drop in P450 reductase apoprotein (p < or = 0.05) in the liver. 3. The increase in hepatic CYP2E1 was associated with increased gene transcription in nuclear run-on experiments. 4. Restoration of renal CYP2E1 to control levels by hormone treatment required both growth hormone and an intact testis, whereas partial restoration of CYP2E1 apoprotein levels in liver was accomplished by growth hormone, but not testosterone. 5. Renal NADPH-dependent cytochrome c reductase activity was restored by growth hormone and testosterone treatment, whereas the hepatic reductase appeared to be regulated by other pituitary hormones. 6. CYP2E1 and P450 reductase appear to be under complex endocrine regulation by pituitary and testicular hormones in a tissue specific manner.

Differential effects of B- and T-lymphocyte mitogens on cytochrome P450 in mice

Lipopolysaccharide (LPS) is widely used as a B-lymphocyte mitogen and is known to depress expression of the cytochrome P450 (P450). However, there have been no studies regarding to the effects of the other mitogens on the expression of P450. This study investigated the effects of mitogens on the constitutive and inducible expression of mouse hepatic P450. Following treatment with B-lymphocyte mitogens, such as LPS and pokeweed mitogen (PWM), hepatic P450 content was reduced. LPS and PWM also suppressed activities of microsomal ethoxyresorufin O-deethylase, pentoxyresorufin O-dealkylase and aniline hydroxylase, a representative activity of P4501A1/2, P4502B1/2 and P4502E1, respectively, in both constitutive and P450 induced mice. However, there was no effect when treated with T-lymphocyte mitogens, such as concanavalin A and phytohemagglutinin. Suppression of P450 expression in the LPS- or PWM-treated mice occurred and was shown to involve a decrease in P450 protein and mRNA levels in liver. These results suggest that suppressive effects of mitogens on the expression of P450 might be different and that B-lymphocyte mitogens selectively depress the expression of P450.

Toxicokinetics of organic solvents: a review of modifying factors

This article reviews, with an emphasis on human experimental data, factors known or suspected to cause changes in the toxicokinetics of organic solvents. Such changes in the toxicokinetic pattern alters the relation between external exposure and target dose and thus may explain some of the observed individual variability in susceptibility to toxic effects. Factors shown to modify the uptake, distribution, biotransformation, or excretion of solvent include physical activity (work load), body composition, age, sex, genetic polymorphism of the biotransformation, ethnicity, diet, smoking, drug treatment, and coexposure to ethanol and other solvents. A better understanding of modifying factors is needed for several reasons. First, it may help in identifying important potential confounders and eliminating negligible ones. Second, the risk assessment process may be improved if different sources of variability between external exposures and target doses can be quantitatively assessed. Third, biological exposure monitoring may be also improved for the same reason.

Effect of pyridine on the expression of cytochrome P450 isozymes in primary rat hepatocyte culture

In vivo administration of pyridine has been shown to increase the activity and content of several forms of cytochrome P450 by transcriptional and posttranscriptional mechanisms. The effect of pyridine on CYP1A and CYP2E1 isozymes was studied in a rat hepatocyte culture model. Hepatocytes were isolated from non-induced rats and seeded onto matrigel-coated dishes and incubated in William's medium E containing 10% fetal calf serum, hormones, and essential metals. Cultures were treated with 0, 10 or 25 mM pyridine for 1-3 days and microsomes were isolated to determine catalytic activity and for immunoblot analysis, and total RNA was isolated for mRNA determinations. CYP2E1 content, CYP2E 1 mRNA, and CYP2E1 catalyzed oxidation of p-nitrophenol declined during culture to values of 3, 30 and 19% that of initial, non-cultured controls by day 3 of culture. Pyridine prevented this decline of CYP2E1 protein and activity such that 60-80% original activity remained after 3 days of culture in the presence of 25 mM pyridine. However, pyridine did not prevent the fall in CYP2E1 mRNA levels, nor did pyridine increase the content or activity of CYP2E1 above initial values of microsomes from freshly isolated hepatocytes. Pyridine increased the content of CYP1A2 and the oxidation of ethoxyresorufin 2-4 fold compared to cultures incubated without pyridine over the 3 day culture period. CYP1A1 levels, which rapidly declined, were induced and maintained in the presence of pyridine. Pyridine increased CYP1A content and activity 2-3 fold over initial values of freshly isolated hepatocytes. These increases were associated with corresponding increases in CYP1A mRNA levels. CYP1A2, but not CYP1A1, mRNA levels increased in the cultures incubated in the absence of pyridine. These results indicate that pyridine has different effects on CYP1A and CYP2E1 in this hepatocyte culture model. Pyridine appears to modulate CYP2E1 levels by posttranscriptional mechanisms as CYP2E1 activity and content were maintained in the presence of pyridine under conditions in which CYP2E1 mRNA levels declined. These mechanisms may involve increased translational efficiency of existing CYP2E1 mRNA or stabilization of CYP2E1 protein against degradation. Pyridine increased CYP1A1 and CYP1A2 content, activity and mRNA levels, either inducing CYP1A transcription or stabilizing CYP1A mRNA. Hepatocyte cultures may be a useful model to study the interaction of pyridine with P450 isozymes and their associated drug-mediated toxicity.

Glycerol increases content and activity of human cytochrome P-4502E1 in a transduced HepG2 cell line by protein stabilization

Glycerol is widely used to stabilize cytochrome P-450 and prevent its transformation to cytochrome P-420. The effect of glycerol on the content and activity of human cytochrome P-4502E1 (CYP2E1) in a HepG2 cell line that stably and constitutively expresses this P-450 was evaluated by immunoassays and oxidation of p-nitrophenol. Addition of 100 to 200 mM glycerol to the culture medium resulted in a 2 1/2- to 3-fold increase in the content and activity of CYP2E1 in microsomes isolated from the cells. Increases could be observed within 4 to 8 hr after addition of glycerol to the culture medium. Glycerol had no effect on the content of cytochrome b5 or activities of NADPH-cytochrome P-450 reductase or NADH-cytochrome b5 reductase. Upon the addition of cycloheximide to stop protein synthesis, CYP2E1 content and activity decreased with apparent half-lives of 6 and 4 hr, respectively. Glycerol prevented or decreased this loss of CYP2E1 content and activity. Labeling CYP2E1 with [35S]methionine, followed by pulse-chase experiments with cold methionine and immunoprecipitation of CYP2E1 indicated a half-life for CYP2E1 of approximately 3 hr. Glycerol increased the half-life to approximately 11 hr. Stabilization of CYP2E1 protein by glycerol was not additive or synergistic with the increase of CYP2E1 by ethanol or 4-methylpyrazole, suggesting that all three agents elevate CYP2E1 by a similar type of mechanism in this model. These results indicate that glycerol can interact with human CYP2E1 to stabilize it against proteolytic degradation, increasing the half-life of the enzyme and thereby elevating the content and activity of CYP2E1.

Characterization of cytochrome P4502E1 turnover in transfected HepG2 cells expressing human CYP2E1

The aim of the present study was to characterize human CYP2E1 turnover and examine the possible proteolytic pathways responsible for the rapid degradation of CYP2E1 in a transfected HepG2 cell line expressing human CYP2E1. Two methods were used to study the CYP2E1 turnover; after addition of cycloheximide, the half-life of the CYP2E1 in the intact cells was about 6 h as detected by PNP catalytic activity assay and immunoblot analysis of apoprotein content. CYP2E1 substrates or ligands such as 4-methylpyrazole, ethanol, glycerol, and dimethyl sulfoxide protected CYP2E1 against this rapid degradation, whereas CCl4 accelerated this process. The second procedure involved pulse-chase experiments after labeling CYP2E1 with [35S]methionine and immunoprecipitation with anti-human CYP2E1 IgG. The half-life of CYP2E1 was about 2.5 h, and the various substrates or ligands modified the turnover process within intact cells as described for the cycloheximide experiments. More than 20 different reagents including antioxidants, physiological metabolites, lysosomal inhibitors, and protease inhibitors were screened for possible effects on CYP2E1 proteolytic degradation. Dibutyryl cAMP had no effect on CYP2E1 activity or turnover. Among those reagents tested so far, the serine protease inhibitor 1-chloro-3-tosylamido-7-amino-2-heptanone hydrochloride exhibited some protection against CYP2E1 degradation. To demonstrate whether the proteasome complex is involved in this process, Czb-Ile-Glu(OtBu)-Ala-leucinal (PSI) as a cell penetrating aldehydic proteasome inhibitor and Czb-Leu-norleucinal (calpeptin inhibitor) as an aldehydic nonproteosomal protease inhibitor were used to examine their effect on both the normal and the CCl4-stimulated CYP2E1 proteolytic degradation pathways. Treatment with PSI at concentrations ranging from 5 to 80 microM resulted in a dose-dependent protection against the loss of both the normal CYP2E1 and the CCl4-modified CYP2E1. The maximum protection by PSI at a concentration of 80 microM after a 12-h chase period was about 60% in cells treated with 2 mM CCl4 or 75% in cells without CCl4 treatment. Calpeptin inhibitor afforded little or no protection against CYP2E1 degradation in the absence or presence of CCl4. PSI did not inhibit CYP2E1 catalytic activity, suggesting that it was not a ligand for CYP2E1. These results indicate that human CYP2E1 has a short half-life span and that substrates can significantly modify its turnover rate in intact HepG2 cells. The proteasome proteolytic pathway may be involved in the degradation process of both the normal and the CCl4-modified human CYP2E1 in this model.

Ethylbenzene modulates the expression of different cytochrome P-450 isozymes by discrete multistep processes

Ethylbenzene (EB) treatment to male Holtzman rats was shown to alter the expression of cytochrome P-450s 1A1, 2B, 2C11, 2E1, and 3A, with several isozymes exhibiting complex multiphasic induction patterns when treated for 1 and 3 days with the alkylbenzene. Male rats were treated with daily i.p. injections of EB for either one or three days, and the effects on P-450 dependent activities, P-450 immunoreactive protein levels and their corresponding mRNA levels were measured. Although levels of P-450 2B, 2C11, 2E1, and 3A were all modulated by EB treatment, each exhibited different temporal characteristics. P-450 2B1/2B2 were induced after a single EB exposure and continued to be elevated after EB treatment for 3 days. However, P-450 2B1 and 2B2 mRNA levels were elevated about 50-fold after a single injection, and returned to control values after continued EB administration. P-450 2C11 expression was decreased to about 45% of controls after either single or repeated EB exposure with corresponding changes being observed in the levels of 2C11 mRNA. P-450 2E1 was induced by EB according to a complex multistep induction pattern. Both P-450 2E1 protein and RNA levels were increased 2-4-fold after a single EB treatment but returned to control values after continued administration. P-450 3A-dependent testosterone 2beta-hydroxylation and P-450 3A immunoreactive protein levels were both increased about 3-fold after a single EB treatment, whereas levels were only elevated 2-fold after EB treatment for 3 days. In contrast, P-450 3A2 mRNA was unaffected by a single EB injection but was increased 3.5-fold with repeated administration. Changes in P-450 3A1/2 were similar to those observed with P-450 3A2, whereas changes in P-450 3A1/23 and 3A23 mRNAs were not detectable. These data indicate that while EB can influence the expression of several P-450 isozymes, the hydrocarbon appears to alter P-450 expression by acting at different regulatory steps.

Selective induction of phase II drug metabolizing enzyme activities by quinolines and isoquinolines

Rats treated with quinoline, and to a lesser extent, isoquinoline (75 mg/kg, daily for 3 days) showed induction of phase II drug metabolizing enzyme activities without inducing either cytochrome P450 concentration or CYP1A-, CYP2B-, CYP2E-, and CYP3A-selective activities. Elevations of UDP-glucuronosyltransferase activities towards 4-nitrophenol, 1-naphthol, and morphine elicited by quinoline (1.9- to 2.7-fold), were greater than those elicited by isoquinoline (1.4- to 1.8-fold). UDP-glucuronosyltransferase activities towards estrone and testosterone were not increased by either compound. Microsomal epoxide hydrolase activity was increased only by quinoline (2.7-fold). NAD(P)H quinone oxidoreductase activity was increased 2-fold by quinoline and isoquinoline. Cytosolic glutathione S-transferase (GST) activity was increased similarly (approximately 20%) by both agents. Similar treatment of rats with either quinine (75 mg/kg) or chloroquine (150 mg/kg) increased 1-naphthol glucuronidation and GST (quinine only) activities. At 75 mg/kg, chloroquine did not affect any phase II enzyme activities but caused a minor elevation of a phase I enzyme, CYP1A; ascertained from an elevation of 7-ethoxyresorufin deethylase activity and a small hypsochromic shift to the absorbance maximum of the cytochrome P450 CO-complex. With quinoline and isoquinoline treatments (n = 14), the correlation coefficients (R) between microsomal epoxide hydrolase and UDP-glucuronosyltransferase activities towards 4-nitrophenol and morphine were 0.96 and 0.92 respectively, suggesting a highly coordinated induction. The highest NAD(P)H quinone oxidoreductase correlations were with microsomal epoxide hydrolase and UDP-glucuronosyltransferase activities towards 4-nitrophenol and morphine (R approximately 0.78). Correlation coefficients between GST and microsomal epoxide hydrolase and NAD(P)H quinone oxidoreductase activities were approximately 0.49. Quinoline and isoquinoline, nitrogen heterocyclic analogs of naphthalene, join the list of simple nitrogen-containing polycyclic aromatic agents capable of selective induction of phase II drug metabolizing enzymes. The position of the single heterocyclic nitrogen atom in the bicyclic ring influences the magnitude and breadth of the induction response. The addition of bulky ring substituents (quinine, chloroquine) reduced the induction response.

Time course for the modulation of hepatic cytochrome P450 after administration of ethylbenzene and its correlation with toluene metabolism

The goal of the present study was to examine the time course for changes in P450 expression and hydrocarbon metabolism after acute treatment with the simple aromatic hydrocarbon ethylbenzene (EB) and to correlate these alterations with the changes observed in alkylbenzene metabolism. Male Holtzman rats were treated with a single intraperitoneal injection of EB, and the effects on specific P450-dependent activities, immunoreactive P450 isozyme levels, and RNA levels were measured at various times after injection. Toluene was used as the test alkylbenzene for examination of the EB-mediated changes on in vitro hydrocarbon metabolism. In untreated rats, toluene was metabolized almost entirely by aliphatic hydroxylation (to benzyl alcohol); however, in EB-treated rats, significant quantities of benzyl alcohol, o-cresol, and p-cresol were produced. Interestingly, 5-10 h after EB treatment, there was a 40% decrease in benzyl alcohol production. By 24 h, rates of benzyl alcohol formation returned to control levels, whereas there was a 7-fold increase in o-cresol and a greater that 50-fold increase in p-cresol production. The changes in the disposition of toluene were then correlated with changes in particular P450 isozymes. Several P450 isozymes were induced after EB administration. P450 2B1/2-dependent testosterone 16 beta-hydroxylation and P450 2B1/2-immunoreactive protein were elevated 30-fold after EB administration, reaching maxima by 24 h and remaining elevated 48 h after exposure. Changes in P450 2B1 and 2B2 RNA preceded those of the proteins. Similar results were observed with P450 1A1. P450 2E1 RNA levels were elevated after a single EB injection. However, the elevation in P450 2E1-dependent activities and immunoreactive protein levels preceded the changes in RNA, suggesting that multiple steps are affected by EB exposure. In contrast to the increases in some isozymes, P450 2C11 protein was rapidly suppressed (within the first 2-10 h) after hydrocarbon exposure, suggestive of a destabilization of the protein. When comparing the changes in P450 isozymes to alterations in toluene metabolism, the immediate suppression in aliphatic hydroxylation of toluene (in the first 5-10 h) was consistent with the decrease in P450 2C11. Subsequent to this effect, P450 2B1/2 and 2E1 were induced, which elevated production of this metabolite to control levels. The increase in the aromatic hydroxylation of toluene to both o, and p-cresol was consistent with the induction of P450s 2B1/2, 2E1, and 1A1.

Ethanol metabolism, cirrhosis and alcoholism

Alcohol-induced tissue damage results from associated nutritional deficiencies as well as some direct toxic effects, which have now been linked to the metabolism of ethanol. The main pathway involves liver alcohol dehydrogenase which catalyzes the oxidation of ethanol to acetaldehyde, with a shift to a more reduced state, and results in metabolic disturbances, such as hyperlactacidemia, acidosis, hyperglycemia, hyperuricemia and fatty liver. More severe toxic manifestations are produced by an accessory pathway, the microsomal ethanol oxidizing system involving an ethanol-inducible cytochrome P450 (2E1). After chronic ethanol consumption, there is a 4- to 10-fold induction of 2E1, associated not only with increased acetaldehyde generation but also with production of oxygen radicals that promote lipid peroxidation. Most importantly, 2E1 activates many xenobiotics to toxic metabolites. These include solvents commonly used in industry, anaesthetic agents, medications such as isoniazid, over the counter analgesics (acetaminophen), illicit drugs (cocaine), chemical carcinogens, and even vitamin A and its precursor beta-carotene. Furthermore, enhanced microsomal degradation of retinoids (together with increased hepatic mobilization) promotes their depletion and associated pathology. Induction of 2E1 also yields increased acetaldehyde generation, with formation of protein adducts, resulting in antibody production, enzyme inactivation, decreased DNA repair, impaired utilization of oxygen, glutathione depletion, free radical-mediated toxicity, lipid peroxidation, and increased collagen synthesis. New therapies include adenosyl-L-methionine which, in baboons, replenishes glutathione, and attenuates mitochondrial lesions. In addition, polyenylphosphatidylcholine (PPC) fully prevents ethanol-induced septal fibrosis and cirrhosis, opposes ethanol-induced hepatic phospholipid depletion, decreased phosphatidylethanolamine methyltransferase activity and activation of hepatic lipocytes, whereas its dilinoleoyl species increases collagenase activity. Current clinical trials with PPC are targeted on susceptible populations, namely heavy drinkers at precirrhotic stages.

Influence of polymorphic N-acetyltransferase phenotype on the inhibition and induction of acetaminophen bioactivation with long-term isoniazid

OBJECTIVE

To determine in patients receiving isoniazid prophylaxis whether an increase in the CYP2E1 dependent formation clearance of acetaminophen (paracetamol) to N-acetyl-p-benzoquinone imine (NAPQI) occurs during a normal 24-hour isoniazid dose interval and whether the interaction is dependent on acetylation status.

METHODS

Acetaminophen elimination kinetics were determined on four different occasions. Ten subjects were assigned to receive acetaminophen either simultaneously with the 8 am dose of isoniazid or 12 hours after the isoniazid dose. One week later, on the last day of isoniazid therapy, subjects received acetaminophen at the alternate time of day. The control phase acetaminophen administrations were repeated 1 and 2 weeks later, following the initial randomization. Isoniazid acetylation (NAT2) genotype was determined by analysis of genomic DNA obtained from peripheral blood leukocytes.

RESULTS

The mean NAPQI formation clearance was inhibited 57% when acetaminophen and isoniazid were coadministered but was unchanged compared with time-matched control when acetaminophen was given 12 hours after the isoniazid dose. However, when data from subjects was segregated according to isoniazid (INH) acetylation phenotype, the mean ratio of NAPQI formation clearances (+INH/-INH) with 8 PM acetaminophen was significantly higher for fast acetylators compared with slow acetylators (1.36 versus 0.68; p = 0.006).

CONCLUSIONS

Fast metabolizers of isoniazid appeared to clear the inducer or inhibitor from the active site of CYP2E1 more rapidly, which resulted in an increased formation of NAPQI 12 hours after the isoniazid dose. In contrast, formation of NAPQI for slow isoniazid metabolizers remained inhibited.

Suppression of rat hepatic cytochrome P450 2E1 expression by isopropyl 2-(1,3-dithioetane-2-ylidene)-2-[N-(4-methyl-thiazol-2-yl)carbamoyl] acetate (YH439), an experimental hepatoprotectant: protective role against hepatic injury

The expression of cytochromes P450 2E1, P450 2B and P450 1A was examined in rat hepatic tissue in response to YH439, an experimental hepatoprotective agent. P450 2E1 metabolic activities relatively specific for P450 2E1 were decreased up to 57% of control activities in the hepatic microsomes prepared from rats treated with YH439 for 3 days. Immunoblot analyses showed that P450 2E1 levels were decreased below the limit of detectability in hepatic microsomes prepared from YH439-treated rats. YH439 at doses from 25 to 100 mg/kg completely suppressed isoniazid-inducible P450 2E1 levels as monitored by both metabolic activities and immunoblot analysis. RNA hybridization analysis revealed that P450 2E1 mRNA levels failed to change after YH439 treatment. These results demonstrate the YH439 effectively suppresses P450 2E1 expression in the absence of transcriptional inactivation. YH439 failed to affect P450 2B1/2 expression, whereas this agent enhanced the hepatic P450 1A1/2 levels. The hepatoprotective effects of YH439 were also examined. Animals treated with CCl4 and ethanol for 9 weeks showed hepatic injury as demonstrated by 2.5- and 2-fold increases in serum alanine aminotransferase and alkaline phosphatase activities, respectively. Concomitant YH439 treatment resulted in a significant protective effect against the experimental hepatic injury. The toxicant-induced elevation in hepatic hydroxyproline level was completely blocked by YH439 treatment. These data indicate that YH439 suppresses the expression of P450 2E1 and protects the liver against chemical-induced hepatic injury and that the selective modulation of detoxifying enzymes by YH439 may contribute to the protection of liver from xenobiotic-induced intoxication.

Maintenance and activation of Cyp2e-1 gene expression in mouse hepatocytes in primary culture

The expression of Cyp2e-1 mRNA and protein was investigated in the C57BL/6NCrj mouse hepatocytes in primary culture, as well as liver and kidney. The mRNA and protein expression in the liver was in the same range in both sexes and was not affected by orchiectomy or ovariectomy. The mRNA expression was enhanced in the kidney of ovariectomized mice, in which the protein contents were not influenced. Orchiectomy decreased the expression of both mRNA and protein. When the hepatocytes were transferred to primary culture, the amounts of the mRNA were not changed within 24 h and about half remained by day 3. However, the expression was low thereafter. The expression of the protein gradually decreased after the start of culture. Dexamethasone showed a potential as an inducer at more than 10(-8) M. Sex hormones increased the expression of this P-450 species a little in culture, but growth hormone did not. These observations indicated that glucocorticoid hormone plays a role in modifying expression of Cyp2e-1 and that the mouse hepatocyte culture is useful for examining its regulation mechanism.

Interaction of 2,4,4'-trichloro-2'-hydroxydiphenyl ether with microsomal cytochrome P450-dependent monooxygenases in rat liver

We studied the effects of 2,4,4'-trichloro-2'-hydroxydiphenyl ether (Irgasan DP300) on the kinetics of the cytochrome P450 (P450)-dependent monooxygenases in rat liver microsomes. The activities of 7-ethoxyresorufin O-deethylase (EROD) and 7-pentoxyresorufin O-depentylase (PROD) in rat liver microsomes exposed to 3-methylcholanthrene (MC) and phenobarbital (PB) respectively, were substantially inhibited by Irgasan DP300. The inhibition profile of EROD was competitive, whereas that of PROD was noncompetitive; the Ki values from Hanes plots were 0.24 and 1.48 microM for EROD and PROD, respectively. Phenacetin O-deethylase (PCOD) and 4-nitrophenol hydroxylase (4NPH) activities in rats exposed to PB were also inhibited by Irgasan DP300, at Ki values lower than those for other microsomes. Irgasan DP300 slightly inhibited testosterone 6 beta-hydroxylase (TS6BH) activities in some microsomes. No effect of Irgasan DP300 on lauric acid omega-hydroxylase (LAOH) activity was evident in any microsomal preparations. These results indicated that Irgasan DP300 inhibits MC- and PB-inducible P450-dependent monoxygenase in vitro competitively or noncompetitively, and that the P450 enzymes of the CYP1A or CYP2B subfamily may contribute to Irgasan DP300 toxicity.

P450 2E1 expression in liver, kidney, and lung of rats treated with single or combined inducers

Ethanol consumption combined with smoking increase the risk of cancer in many tissues. Such a mechanism implies the involvement of cytochrome P450 alcohol (CYP2E1), which is regulated by numerous xenobiotics. The combination of P450 2E1 inducers (acetone or pyridine) and 3-methylcholanthrene during rat treatment was shown to decrease the liver P450 2E1 content while it enhanced its expression in kidney. It is suggested that this differential tissue response helps explain the organotropy of nitrosamine carcinogenicity.

Induction of hepatic cytochrome P4502E1 in rats by acetylsalicylic acid or sodium salicylate

Studies were conducted on the mechanism of the ethanol-inducible cytochrome P450 (cytochrome P4502E1, CYP 2E1) induction by acetylsalicylic acid (ASA) or its metabolite salicylate (SAL). Many exogenous inducers of CYP 2E1 seem to increase CYP 2E1 by post-transcriptional activation without elevation of its mRNA level. Administration of a single high dose of ASA or SAL produces a significant increase in the activity of the hepatic microsomal p-nitrophenol hydroxylase in rats. Pretreatment of ASA-treated rats with a blocker of mRNA transcription, actinomycin D, or a blocker of protein synthesis, cycloheximide, markedly suppressed this enhanced activity of microsomal p-nitrophenol hydroxylase. The CYP 2E1 mRNA levels in livers of control rats and rats treated with ASA or SAL were measured by Northern blot analysis. Significantly elevated CYP 2E1 mRNA levels were measured in livers of treated rats compared with mRNA amounts of the control group. These data suggest that mRNA elevation seems to be characteristic for ASA induction, while other inducing agents show different patterns and mechanisms of activation.

Role of oxidative stress and antioxidant therapy in alcoholic and nonalcoholic liver diseases

The main pathway for the hepatic oxidation of ethanol to acetaldehyde proceeds via ADH and is associated with the reduction of NAD to NADH; the latter produces a striking redox change with various associated metabolic disorders. NADH also inhibits xanthine dehydrogenase activity, resulting in a shift of purine oxidation to xanthine oxidase, thereby promoting the generation of oxygen-free radical species. NADH also supports microsomal oxidations, including that of ethanol, in part via transhydrogenation to NADPH. In addition to the classic alcohol dehydrogenase pathway, ethanol can also be reduced by an accessory but inducible microsomal ethanoloxidizing system. This induction is associated with proliferation of the endoplasmic reticulum, both in experimental animals and in humans, and is accompanied by increased oxidation of NADPH with resulting H2O2 generation. There is also a concomitant 4- to 10-fold induction of cytochrome P4502E1 (2E1) both in rats and in humans, with hepatic perivenular preponderance. This 2E1 induction contributes to the well-known lipid peroxidation associated with alcoholic liver injury, as demonstrated by increased rates of superoxide radical production and lipid peroxidation correlating with the amount of 2E1 in liver microsomal preparations and the inhibition of lipid peroxidation in liver microsomes by antibodies against 2E1 in control and ethanol-fed rats. Indeed, 2E1 is rather "leaky" and its operation results in a significant release of free radicals. In addition, induction of this microsomal system results in enhanced acetaldehyde production, which in turn impairs defense systems against oxidative stress. For instance, it decreases GSH by various mechanisms, including binding to cysteine or by provoking its leakage out of the mitochondria and of the cell. Hepatic GSH depletion after chronic alcohol consumption was shown both in experimental animals and in humans. Alcohol-induced increased GSH turnover was demonstrated indirectly by a rise in alpha-amino-n-butyric acid in rats and baboons and in volunteers given alcohol. The ultimate precursor of cysteine (one of the three amino acids of GSH) is methionine. Methionine, however, must be first activated to S-adenosylmethionine by an enzyme which is depressed by alcoholic liver disease. This block can be bypassed by SAMe administration which restores hepatic SAMe levels and attenuates parameters of ethanol-induced liver injury significantly such as the increase in circulating transaminases, mitochondrial lesions, and leakage of mitochondrial enzymes (e.g., glutamic dehydrogenase) into the bloodstream. SAMe also contributes to the methylation of phosphatidylethanolamine to phosphatidylcholine. The methyltransferase involved is strikingly depressed by alcohol consumption, but this can be corrected, and hepatic phosphatidylcholine levels restored, by the administration of a mixture of polyunsaturated phospholipids (polyenylphosphatidylcholine). In addition, PPC provided total protection against alcohol-induced septal fibrosis and cirrhosis in the baboon and it abolished an associated twofold rise in hepatic F2-isoprostanes, a product of lipid peroxidation. A similar effect was observed in rats given CCl4. Thus, PPC prevented CCl4- and alcohol-induced lipid peroxidation in rats and baboons, respectively, while it attenuated the associated liver injury. Similar studies are ongoing in humans.

Induction of cytochrome P-450 in the Norway rat, Rattus norvegicus, following exposure to potential environmental contaminants

Cytochrome P-450 (CYP) induction (consisting of increases in cellular RNA and protein content and associated catalytic activities) occurs predominantly in the liver, but also in small intestine, lung, kidney, and placenta, of Norway rats (Rattus norvegicus) exposed to certain types of potential environmental contaminants. The specific isoform(s) induced in the rat and the magnitudes of the increases observed depend upon the chemical nature of the xenobiotic. For instance, the predominant isoforms induced by nonhalogenated polycyclic aromatic hydrocarbons, such as petroleum derivatives and coal-tar constituents such as the benzopyrenes and the anthracenes, are those of the CYP1A subfamily. Polyhalogenated aromatic hydrocarbons, such as the halogenated dibenzodioxins, dibenzofurans, and biphenyls, may cause the induction of predominantly the CYP1A subfamily, predominantly the CYP2B subfamily, or mixed CYP1A- and CYP2B-type induction, depending upon the halogen substitution pattern. In contrast, the chlorinated hydrocarbon pesticides, such as DDT, dieldrin, chlordane, and mirex, cause almost exclusively the induction of isoforms of the CYP2B (and to a lesser extent the CYP3A) subfamilies. The commonly employed plasticizing agent di-(2-ethylhexyl)phthalate elicits predominantly induction of the CYP4A subfamily. Those xenobiotics that would be expected to be the most pervasive environmental contaminants are typically those that have also been found to cause the most profound CYP induction responses. Such chemicals are extremely lipophilic and tend to accumulate in animal tissues, especially fatty tissues such as the liver. The hepatic CYP induction response to such potential environmental contaminants is typical of the animals' response to lipophilic xenobiotics in general, and serves as a mechanism by which the excretion of such compounds from the body is facilitated.

Induction mechanisms of cytochrome P450 2E1 in liver: interplay between ethanol treatment and starvation

Chronic ethanol exposure causes marked induction of the ethanol-inducible cytochrome P450 (CYP) 2E1 isozyme in the centrilobular liver region, where alcoholic damage commonly is initiated. In contrast to most other CYP forms, which are ligand-activated at the transcriptional level, ethanol induction of CYP2E1 has been found to be post-translational. However, transcriptional activation of the CYP2E1 gene was recently described in fed animals maintained at very high ethanol levels. To further evaluate mechanisms of ethanol-mediated CYP2E1 induction we compared the effect of short-term heavy-ethanol treatment and fasting on CYP2E1 mRNA, protein and catalytic activity. High blood-ethanol levels (20-70 mM) were maintained for 3 days by regular alcohol intubations to fed or fasted rats. During this period, the amount of liver CYP2E1 apoprotein increased a maximum of 20-fold and catalytic activity 16-fold, both in fed and fasted animals, whereas starvation alone caused only a 4- to 5-fold increase. By comparison, the amount of CYP2E1 mRNA, as assayed both by Northern blot and slot blot, was significantly increased (5- to 6-fold) by ethanol only in fasted rats; this increase was smaller than that observed after fasting alone (8- to 9-fold). Analysis of cell lysates isolated from the periportal and perivenous region revealed that the increase in CYP2E1 mRNA by fasting occurred in the perivenous region. Thus no evidence was obtained for an increased pretranslational CYP2E1 gene expression as a consequence of the continuous presence of ethanol at intoxicating levels for 3 days. CYP2E1 mRNA elevation seems to be strongly associated with starvation while alcohol treatment increases the amount of enzyme, primarily by ligand-dependent stabilization of the synthesized protein. Our results indicate that transcriptional activation of CYP2E1 requires the long-term presence of highly intoxicating ethanol levels. It is conceivable that such activation occurs via indirect physiological responses related to those triggered by starvation.

Pyridine induction of Sprague-Dawley rat renal cytochrome P4502E1: immunohistochemical localization and quantitation

Previous research has shown that i.p. injection of rats with pyridine results in a significant increase in immunoreactive renal cytochrome P4502E1 (alcohol-inducible form) in a dose- and time-dependent manner. However, the cellular location of renal P4502E1 in rats was not reported. Thus, it was not known whether the pyridine-induced increase in renal P4502E1 resulted from increased production of the enzyme in cells which normally express P4502E1 or from de novo expression in cells normally devoid of the protein. To address these questions, rats were injected i.p. with either 200 mg pyridine/kg body wt./day for 1, 2, 3, or 4 days (n = 2/group) or injected once with an equal volume of sterile, pyrogen-free saline (control group; n = 2). Kidney tissue samples from saline- and pyridine-exposed rats were processed by light microscopy and were immunochemically stained to detect rat cytochrome P4502E1. Most of the immunoreactive P4502E1 was located within renal cortical epithelial cells lining proximal and distal tubules of the cortex with lesser--but consistent--amounts present in tubular epithelial cells within the inner and outer medulla. Pyridine exposure resulted in a 2-3-fold increase in P4502E1 immunoreactivity in proximal cortical tubules surrounding glomeruli and cortical blood vessels. The results of this study demonstrate a cell-specific distribution of cytochrome P4502E1 within the rat kidney and indicate that pyridine exposure results in a selective induction of immunoreactive P4502E1 in tubule epithelial cells which constitutively express this enzyme. The results of this study provide a morphologic basis for interpreting cell-specific nephrotoxicity due to xenobiotics that are biotransformed to toxic metabolites by renal P4502E1.

Pre-translational regulation of cytochrome P450 genes is responsible for disease-specific changes of individual P450 enzymes among patients with cirrhosis

We have recently reported that disease-specific differential alterations in the hepatic expression of xenobiotic-metabolizing cytochrome P450 (CYP P450) enzymes occur in patients with advanced liver disease. In order to determine whether the observed changes in CYP proteins are modulated at pre- or post-translational levels, we have now examined the hepatic levels of mRNA for CYPs 1A2, 2C9, 2E1 and 3A4 by solution hybridization in the same livers of 20 controls (surgical waste from histologically normal livers), 32 cases of hepatocellular and 18 of cholestatic severe chronic liver disease. CYP1A2 mRNA and CYP1A immunoreactive protein were both reduced in livers with hepatocellular and cholestatic types of cirrhosis. In contrast, CYP3A4 mRNA and protein were reduced only in livers from patients with hepatocellular diseases. For 1A2 and 3A4 there were significant correlations between mRNA species and the respective protein contents (rS1A2 = 0.74, rS3A4 = 0.64, P < 0.0001). CYP2C9 mRNA was reduced in patients with both cholestatic and hepatocellular types of liver disease, but 2C protein was reduced only in patients with cholestatic dysfunction. The correlation between CYP2C9 mRNA and protein, was also significant (rs = 0.36, P < 0.005) but mRNA levels accounted for only 13% of the variability in protein rankings. This is probably a consequence of other CYP2C proteins apart from 2C9 being detected by the anti-2C antibody. CYP2E1 mRNA and protein were reduced in patients with cholestatic liver disease, but in hepatocellular disease the expression of only CYP2E1 mRNA was decreased. CYP2E1 mRNA was significantly correlated with CYP2E1 protein but accounted for only 18% of the variability in protein rankings (rs = 0.43, P < 0.0005). Taken collectively these data indicate that the disease-specific alterations of xenobiotic-metabolizing CYP enzymes among patients with cirrhosis is due, at least in part, to pre-translational mechanisms. The lack of a strong correlation between CYP2E1 mRNA and protein suggests that this gene, like its rat orthologue, may be subject to pre-translational as well as translational and/or post-translational regulation.

Relationship between hydrocarbon structure and induction of P450: effect on RNA levels

1. Exposure to simple aromatic hydrocarbons has been shown to induce P450-dependent activities and the expression of particular P450 isozymes in a manner related to the molecular structure of the inducing hydrocarbon. In an attempt to identify the structural relationship controlling P450 induction, the effect of hydrocarbon treatment on the RNA levels for specific P450 isozymes was examined. 2. Rats were treated with daily injections of hydrocarbons (benzene, toluene, ethylbenzene, n-propylbenzene, m- and p-xylene) for 3 days, and the effects on specific RNA levels were examined by Northern blot hybridization. 3. Although P4502B1 mRNA was not elevated after hydrocarbon treatment, a significant elevation in 2B2 mRNA was observed after exposure to the larger aromatic hydrocarbons, ethylbenzene and m-xylene. It is interesting to note that despite the substantial elevation of P4502B protein levels, only a small elevation of P4502B1 and 2B2 RNA was observed. 4. P4502C11 mRNA was only suppressed by ethylbenzene administration, despite the depression of 2C11 protein levels by several hydrocarbons. 5. P4501A1 mRNA was not detectable and 2E1 mRNA was not changed by any aromatic hydrocarbon treatment investigated in this study. 6. The data indicate that the levels of mRNA species for a number of P450 isozymes are differentially regulated by exposure to hydrocarbons, and that small changes in hydrocarbon size or isomeric structure can influence the levels of these mRNA species.

The effect of inducers and inhibitors of urethane metabolism on its in vitro and in vivo metabolism in rats

The activation of urethane (ethyl carbamate) is important in its exerting its carcinogenic effect. Rats were treated with inducers and inhibitors of urethane metabolism, and the conversion of [carbonyl-14C]urethane to 14CO2 in vivo was measured. The cytochrome P-450 inducers, phenobarbital and beta-naphthoflavone, and esterase inhibitor, paraoxon, were without effect while the CYP2E1 inhibitor, diethyldithiocarbamate, decreased metabolism to about 3% of control. Ethanol administered acutely inhibited urethane metabolism. Pyridine, shown previously to enhance this metabolism in microsomal preparations, greatly inhibited it in vivo. The discordant results between the in vitro and in vivo studies may be related to the presence of pyridine acting as an inhibitor in whole animals and suggest that caution is needed in extrapolating from in vitro results to in vivo implications.

Post-transcriptional action of ACTH in the control of P450c17 expression in rabbit adrenal glands

Stimulation of transcription of the CYP17 gene by ACTH has been demonstrated previously by others using cultures of adrenal tissue from several species. In the present investigations we have demonstrated from measurements in pooled rabbit adrenal glands that after 4 or 6 days of ACTH injections no difference in amounts of CYP17 mRNA per microgram of total adrenal RNA was observed between ACTH and control animals. While the total amount of CYP17 mRNA per adrenal increased 1.4- to 1.7-fold over the injected and non-injected controls due to an increase in total RNA in the ACTH-stimulated adrenals, Western analysis of adrenal microsomal protein demonstrated that the amount of adrenal P450c17 protein, the product of the CYP17 gene, increased over 50-fold. The data show that ACTH is acting at both transcriptional and post-transcriptional loci to increase the amount of rabbit adrenal P450c17 and that the greater effect is post-transcriptional.

Characterization of cytochrome P450 2E1 induction in a rat hepatoma FGC-4 cell model by ethanol

The hepatic microsomal ethanol-oxidizing system (MEOS) has been well characterized as an important pathway in ethanol metabolism. Cytochrome P450 2E1 (CYP 2E1), the principal component of MEOS, is ethanol inducible and has been implicated in hepatotoxicity associated with alcohol abuse and exposure to organic solvents. Results of chronic in vivo experiments have shown that ethanol induction of hepatic CYP 2E1 occurs by a two-step mechanism. The first step of induction is associated with low blood alcohol concentrations (BACs) and appears to be post-transcriptional, whereas high BACs observed in step-two induction are associated with increased CYP 2E1 gene transcription. The mechanisms underlying these induction steps are under intense investigation. Progress in this area has been limited due to lack of hepatic cell culture models that express CYP 2E1. We report here an in vitro tissue culture cell model, the FGC-4 hepatoma cell line, that exhibits basal levels of CYP 2E1 apoprotein that are inducible by ethanol treatment. Total cellular RNA and microsomal fractions were isolated from control or ethanol-treated confluent cells, and CYP 2E1 mRNA and apoprotein levels were characterized by northern blot or immunoblot analysis, respectively. Initial experiments on isolated microsomes revealed detectable levels of CYP 2E1 apoprotein in control cells that were induced 5-fold in cells treated with 100 mM ethanol for 24 hr. Concentration-response experiments demonstrated that the maximal 24-hr induction in CYP 2E1 apoprotein level was 5-fold and was attained at a concentration of 10 mM ethanol. Interestingly, while the steady-state mRNA levels encoding CYP 2E1 were detectable, they remained unchanged in identically treated cells. Furthermore, there was no observed increase in CYP 2E1 mRNA levels in an extended time course to 72 hr or at higher alcohol concentrations (up to 1500 mM), providing preliminary evidence that the induction is post-transcriptional. The time course of CYP 2E1 apoprotein induction by exposure to 100 mM ethanol demonstrated maximal induction at 8 hr. Measurement of CYP 2E1 apoprotein levels after removal of ethanol from pretreated cells demonstrated the half-life of the apoprotein to be 12.7 hr, in good agreement with previous reports using primary hepatocytes. The half-life of the induced protein after ethanol removal in the presence of cyclohexamide (10 micrograms/mL) was biphasic with a rapid 1.8 hr first phase followed by a slower 44.7 hr second phase.(ABSTRACT TRUNCATED AT 400 WORDS)

Temporal changes in P-450 2E1 expression with continued ethylbenzene exposure

The goal of this study was to examine the effect of duration of ethylbenzene exposure on cytochrome P-450-dependent activities. Male rats were treated with ethylbenzene by intraperitoneal injection for either 1 or 3 days, and microsomal preparations were examined for changes in the microsomal proteins and activities as well as the expression of specific P-450 isozymes. Two general patterns of induction were evident when different P-450-dependent activities were examined. (i) Cytochrome P-450 2B-dependent activities (e.g., p-nitroanisole demethylation, benzphetamine demethylation, and aromatic toluene hydroxylations) were induced both after 1 and 3 days of ethylbenzene exposure. (ii) Cytochrome P-450 2E1-dependent activities (e.g., N,N-dimethylnitrosamine demethylation and aniline hydroxylation) were induced after treatment with ethylbenzene for one day; however, after 3 days of ethylbenzene treatment these activities returned to control levels. Changes in these activities were consistent with changes in the levels of specific P-450 isozymes as determined by immunoblotting. Cytochrome P-450 2B levels were increased and P-450 2C11 levels were suppressed at both 1 and 3 days of ethylbenzene exposure. A temporal response in P-450 2E1 expression was observed, with P-450 2E1 levels increasing after a single ethylbenzene injection and returning to controls after administration of the hydrocarbon for 3 days. Rats were also subjected to a pair-feeding regimen to determine whether these effects were related to altered dietary status in ethylbenzene-treated rats. Neither P-450-dependent activities nor immunoreactive protein levels were altered in pair-fed rats. These results demonstrate that prolonging the duration of hydrocarbon exposure can produce differential effects on the expression of P-450 2E1, with levels being elevated after acute hydrocarbon administration, but not after more prolonged hydrocarbon exposure.

Alcohol and the liver: 1994 update

This article reviews current concepts on the pathogenesis and treatment of alcoholic liver disease. It has been known that the hepatotoxicity of ethanol results from alcohol dehydrogenase-mediated excessive generation of hepatic nicotinamide adenine dinucleotide, reduced form, and acetaldehyde. It is now recognized that acetaldehyde is also produced by an accessory (but inducible) microsomal pathway that additionally generates oxygen radicals and activates many xenobiotics to toxic metabolites, thereby explaining the increased vulnerability of heavy drinkers to industrial solvents, anesthetics, commonly used drugs, over-the-counter medications, and carcinogens. The contribution of gastric alcohol dehydrogenase to the first-pass metabolism of ethanol and alcohol-drug interactions is discussed. Roles for hepatitis C, cytokines, sex, genetics, and age are now emerging. Alcohol also alters the degradation of key nutrients, thereby promoting deficiencies as well as toxic interactions with vitamin A and beta carotene. Conversely, nutritional deficits may affect the toxicity of ethanol and acetaldehyde, as illustrated by the depletion in glutathione, ameliorated by S-adenosyl-L-methionine. Other "supernutrients" include polyunsaturated lecithin, shown to correct the alcohol-induced hepatic phosphatidylcholine depletion and to prevent alcoholic cirrhosis in nonhuman primates. Thus, a better understanding of the pathology induced by ethanol is now generating improved prospects for therapy.

Inhibition of cytochrome P4502E1 expression by organosulfur compounds allylsulfide, allylmercaptan and allylmethylsulfide in rats

Cytochrome P4502E1 (CYP2E1) is active in both detoxication and activation of small organic molecules. The effects of organosulfur compounds including allylsulfide (AS), allylmercaptan (AM) and allylmethylsulfide (AMS) on the expression of CYP2E1 were examined in rats. 4-Nitrophenol, aniline hydroxylase and N-nitrosodimethylamine demethylase activities, the rates of which represent the level of CYP2E1, decreased in hepatic microsomes isolated from rats treated with AS in a time-dependent manner by 45% to 90%, as compared to control. Pyrazine-induced hepatic microsomes exhibited approximately 5-fold increases in CYP2E1-catalysed metabolic activities, whereas the hepatic microsomes obtained after treatment of animals with both AS and pyrazine showed rates comparable to or less than those in control microsomes. AM or AMS suppressed constitutive and pyrazine-inducible levels of CYP2E1 similarly to AS. Immunoblot analyses of hepatic microsomes, using an anti-CYP2E1 antibody, showed that AS, AM and AMS significantly suppressed constitutive levels of CYP2E1 apoprotein after 24, 48 and 72 hr. Time-dependent induction of CYP2E1 by pyrazine was also completely blocked by treatment of animals with AS throughout the experimental period, as evidenced by immunoblot analysis. The levels of CYP2E1 apoprotein in the hepatic microsomes isolated from animals treated with both AM and pyrazine, or with both AMS and pyrazine were comparable to those in control hepatic microsomes at days 1-3 post-treatment. Treatment of rats with each of these organosulfur compounds caused no significant changes in the levels of CYP2E1 mRNA, as assessed by slot and northern blot analyses, suggesting that post-transcriptional regulation may be associated with the suppression of CYP2E1 apoprotein levels. The results of metabolic activities, immunoblot analyses and RNA blot analyses demonstrated that these organosulfur compounds are effective in suppressing constitutive and inducible expression of CYP2E1.

Mechanisms of ethanol-drug-nutrition interactions

Mechanisms of the toxicologic manifestations of ethanol abuse are reviewed. Hepatotoxicity of ethanol results from alcohol dehydrogenase-mediated excessive hepatic generation of nicotinamide adenine dinucleotide and acetaldehyde. It is now recognized that acetaldehyde is also produced by an accessory (but inducible) pathway, the microsomal ethanol-oxidizing system, which involves a specific cytochrome P450. It generates oxygen radicals and activates many xenobiotics to toxic metabolites, thereby explaining the increased vulnerability of heavy drinkers to industrial solvents, anesthetics, commonly used drugs, over-the-counter medications and carcinogens. The contribution of gastric alcohol dehydrogenase to the first pass metabolism of ethanol and alcohol-drug interactions is now recognized. Alcohol also alters the degradation of key nutrients, thereby promoting deficiencies as well as toxic interactions with vitamin A and beta-carotene. Conversely, nutritional deficits may affect the toxicity of ethanol and acetaldehyde, as illustrated by the depletion in glutathione, ameliorated by S-adenosyl-L-methionine. Other supernutrients include polyenylphosphatidylcholine, shown to correct the alcohol-induced hepatic phosphatidylcholine depletion and to prevent alcoholic cirrhosis in non-human primates. Thus, a better understanding of the pathology induced by ethanol has now generated improved prospects for therapy.

Translational activation of ethanol-inducible cytochrome P450 (CYP2E1) by isoniazid

The molecular mechanism of ethanol-inducible cytochrome P450(CYP2E1) induction by isoniazid was studied and compared to that of pyridine, an inducer of CYP2E1. Aniline hydroxylase and immunoreactive CYP2E1 protein were significantly induced by isoniazid without or with only slight activation of other cytochromes P450. In contrast, pyridine increased the activities of a broad range of P450s. The effects of two structural analogs of isoniazid, isonicotinamide and isonicotinic acid were also tested and found to have a markedly decreased ability to induce CYP2E1. The induction of CYP2E1 by isoniazid was not accompanied by an increased level of CYP2E1 mRNA, and was completely blocked by pretreatment with cycloheximide or sodium fluoride, inhibitors of mRNA translation. These data thus suggest that CYP2E1 induction by isoniazid is due to activation of CYP2E1 mRNA translation and that the hydrazide group on the pyridine ring of isoniazid is important both in the selective induction of CYP2E1 and for magnitude of effect.

Induction of P4502E1 by acetone in isolated rabbit hepatocytes. Role of increased protein and mRNA synthesis

The molecular mechanism(s) underlying induction of the hepatic microsomal cytochrome P4502E1 (2E1) by xenobiotics (e.g. ethanol and acetone) is controversial. Proposed mechanisms include increased rates of enzyme synthesis due to elevated 2E1 mRNA levels, enhanced translation of pre-existing mRNA, or stabilization of 2E1 protein. To further assess which, if any, of these events predominates during the initial stages of 2E1 protein induction, we investigated the effects of acetone treatment on 2E1 content in cultured rabbit hepatocytes, an in vitro system that allows for precise control of the cellular mileau. Hepatocytes harvested from female rabbits and plated on plastic dishes with serum-supplemented medium were 90-100% viable for at least 48 hr in culture. Analysis of immunoreactive 2E1 content and aniline hydroxylase activity in microsomes isolated from hepatocytes cultured for up to 24 hr revealed that 2E1 expression was equal to that of microsomes from unplated cells and by 48 hr of culture, 2E1 levels decreased by only 35%. Moreover, microsomes isolated from cells exposed to 17 mM acetone for 24 hr exhibited a 53 and 62% increase in aniline hydroxylase activity and 2E1 content, respectively, compared to untreated cells. To explain these increases, the rate of 2E1 protein synthesis was determined in untreated cells or in cells treated with 17 mM acetone by first exposing hepatocytes to medium supplemented with 35S-labeled methionine and cysteine ([35S]Met/Cys) and subsequently assessing radiolabel incorporation into 2E1 protein. While no difference was found between untreated and acetone-treated cells in the incorporation of [35S]Met/Cys into trichloracetic acid-precipitable microsomal proteins, immunoaffinity purification of 2E1 revealed that incorporation of 35S-labeled amino acids specifically into 2E1 was elevated by acetone to 200% of control values. Treatment of hepatocytes with the transcriptional inhibitor, alpha-amanitin, markedly inhibited this acetone-mediated increase in [35S]Met/Cys incorporation into 2E1. Analysis of hepatocyte RNA revealed that acetone increased 2E1 mRNA to 130 and 160% of control levels at 6 and 24 hr, respectively, and that these increases were prevented by pretreatment with alpha-amanitin. Our results indicate that acetone increases 2E1 protein levels in cultured rabbit hepatocytes by stimulating its rate of de novo synthesis. Since this increase in 2E1 synthesis stems, at least in part, from the acetone-mediated enhancement of hepatocyte 2E1 mRNA content and is inhibitable by alpha-amanitin, transcriptional activation of the rabbit CYP2E1 gene is apparently involved in the induction of 2E1 protein by acetone.