Compendium of gene expression profiles comprising a baseline model of the human liver drug metabolism transcriptome

Oligonucleotide microarrays were used to study the variability of pharmacokinetics and drug metabolism (PKDM)-related gene expression in 75 normal human livers. The objective was to define and use absorption, distribution, metabolism and excretion (ADME) gene expression variability to discern co-regulated genes and potential surrogate biomarkers of inducible gene expression. RNA was prepared from donor tissue and hybridized on Agilent microarrays against an RNA mass balanced pool from all donors. Clustering of PKDM gene sets revealed donors with distinct patterns of gene expression that grouped genes known to be regulated by the nuclear receptor, pregnane X-receptor (PXR). Fold range metrics and frequency distributions from the heterogeneous human population were used to define the variability of individual PKDM genes in the 75 human livers and were placed in context by comparing expression data with basal ADME gene expression variability in an inbred and diet/environment controlled population of 27 Rhesus livers. The most variable genes in the hepatic transcriptome were mainly related to drug metabolism, intermediary metabolism, inflammation and cell cycle control. Unique patterns of expression across 75 individuals of inducible ADME gene expression allowed their expression to be correlated with the expression of many other genes. Correlated genes for AhR, CAR and PXR responsive genes (CYP1A2, CYP2B6 and CYP3A4) were identified that may be co-regulated and, therefore, provide clues to the identity of surrogate gene or protein markers for CYP induction. In conclusion, microarrays were used to define the variable expression of hepatic ADME genes in a diverse human population, the expression variability of ADME genes was compared with the expression variability in an inbred population of Rhesus monkeys, and genes were defined that may be co-regulated with important inducible CYP genes.

Testosterone, 7-benzyloxyquinoline, and 7-benzyloxy-4-trifluoromethyl-coumarin bind to different domains within the active site of cytochrome P450 3A4

Testosterone, 7-benzyloxyquinoline, and 7-benzyloxy-4-trifluoromethyl-coumarin, marker substrates for cytochrome P450 3A4 are commonly used within the pharmaceutical industry to screen new chemical entities as inhibitors of CYP3A4 in a high-throughput manner to predict the potential for drug-drug interactions. However, it has been observed that inhibition data obtained with a given CYP3A4 probe substrate may not correlate well with results from a different probe. As a consequence, the choice of the probe compound becomes an important consideration in such screens. In the present study, kinetic interactions between either two of the above three substrates were evaluated, and three-dimensional nonlinear regression analysis was performed to understand the kinetic mechanisms of drug interaction. Our results demonstrate that the kinetic interaction between each pair of substrates does not appear to be competitive and that the interactions are characterized by an unchanged or a decrease in both apparent K(m) (a = 0.21-0.72, a change of K(m) in the absence of the effector) and V(max) (alpha and beta = 0.09-0.75, changes of V(max) in the absence of the effector). These data suggest that 1) the three substrates bind to different domains; 2) at least two substrates can coexist in the active site of CYP3A4; and 3) the two bound substrates interact kinetically with each other (e.g., through steric hindrance), thereby leading to a change in both apparent kinetic parameters and partial inhibition. Selection of multiple substrates, which are shown not to be competitive, is necessary to accurately predict CYP3A4 inhibition and the potential for drug-drug interaction.

An invasive cleavage assay for direct quantitation of specific RNAs

RNA quantitation is becoming increasingly important in basic, pharmaceutical, and clinical research. For example, quantitation of viral RNAs can predict disease progression and therapeutic efficacy. Likewise, gene expression analysis of diseased versus normal, or untreated versus treated, tissue can identify relevant biological responses or assess the effects of pharmacological agents. As the focus of the Human Genome Project moves toward gene expression analysis, the field will require a flexible RNA analysis technology that can quantitatively monitor multiple forms of alternatively transcribed and/or processed RNAs (refs 3,4). We have applied the principles of invasive cleavage and engineered an improved 5'-nuclease to develop an isothermal, fluorescence resonance energy transfer (FRET)-based signal amplification method for detecting RNA in both total RNA and cell lysate samples. This detection format, termed the RNA invasive cleavage assay, obviates the need for target amplification or additional enzymatic signal enhancement. In this report, we describe the assay and present data demonstrating its capabilities for sensitive (<100 copies per reaction), specific (discrimination of 95% homologous sequences, 1 in > or =20,000), and quantitative (1.2-fold changes in RNA levels) detection of unamplified RNA in both single- and biplex-reaction formats.

Halothane-induced liver injury in outbred guinea pigs: role of trifluoroacetylated protein adducts in animal susceptibility

Halothane causes a mild form of liver injury in guinea pigs that appears to model the hepatotoxicity seen in approximately 20% of patients treated with this drug. In previous studies, it was concluded that the increased susceptibility of some outbred guinea pigs to halothane-induced liver injury is not caused by their inherent ability to metabolize halothane to form toxic levels of trifluoroacetylated protein adducts in the liver. In this study, we reevaluated the role of trifluoroacetylated protein adducts in halothane-induced liver injury in guinea pigs. Male outbred Hartley guinea pigs were treated with halothane intraperitoneally. On the basis of serum alanine aminotransferase levels and liver histology, treated animals were designated as being susceptible, mildly susceptible, or resistant to halothane. Immunoblot studies with the use of anti-trifluoroacetylated antibodies showed that susceptible guinea pigs for the most part had higher levels of trifluoroacetylated protein adducts in the liver 48 h after treatment with halothane than did less susceptible animals. In support of this finding, the level of trifluoroacetylated protein adducts detected immunochemically in the sera of treated guinea pigs correlated with sera levels of alanine aminotransferase activity. In addition, the levels of cytochrome P450 2A-related protein but not those of other cytochrome P450 isoforms, measured by immunoblot analysis with isoform-specific antibodies, correlated with the amount of trifluoroacetylated protein adducts detected in the livers of guinea pigs 8 h after halothane administration. The results of this study indicate that the susceptibility of outbred guinea pigs to halothane-induced liver injury is related to an enhanced ability to metabolize halothane in the liver to form relatively high levels of trifluoroacetylated protein adducts. They also suggest that cytochrome P450 2A-related protein might have a major role in catalyzing the formation of trifluoroacetylated protein adducts in the liver of susceptible guinea pigs. Similar mechanisms may be important in humans.

In-vitro metabolism of celecoxib, a cyclooxygenase-2 inhibitor, by allelic variant forms of human liver microsomal cytochrome P450 2C9: correlation with CYP2C9 genotype and in-vivo pharmacokinetics

In-vitro studies were conducted to assess the impact of CYP2C9 genotype on the metabolism (methyl hydroxylation) and pharmacokinetics of celecoxib, a novel cyclooxygenase-2 inhibitor and CYP2C9 substrate. When compared to cDNA-expressed wild-type CYP2C9 (CYP2C9*1), the Vmax/Km ratio for celecoxib methyl hydroxylation was reduced by 34% and 90% in the presence of recombinant CYP2C9*2 and CYP2C9*3, respectively. These data indicated that the amino acid substitution at position 359 (Ile to Leu) elicited a more pronounced effect on the metabolism of celecoxib than did a substitution at position 144 (Arg to Cys). The Vmax/Km ratio was also decreased in microsomes of livers genotyped CYP2C9*1/*2 (47% decrease, mean of two livers), or CYP2C9*1/*3 (59% decrease, one liver). In all cases, these changes were largely reflective of a decrease in Vmax, with a minimal change in Km. Based on simulations of the in-vitro data obtained with the recombinant CYP2C9 proteins, it was anticipated that the pharmacokinetics of celecoxib (as a much as a five-fold increase in plasma AUC) would be altered (versus CYP2C9*1/*1 subjects) in subjects genotyped heterozygous or homozygous for the CYP2C9*2 (Cys144) or CYP2C9*3 (Leu359) allele. In a subsequent clinical study, the AUC of celecoxib was increased (versus CYP2C9*1/*1 subjects) approximately 2.2-fold (range, 1.6-3-fold) in two CYP2C9*1/*3 subjects and one CYP2C9*3/*3 subject receiving a single oral dose (200 mg) of the drug. In contrast, there was no significant change in celecoxib AUC in two subjects genotyped CYP2C9*1/*2.

Substrate inhibition kinetics for cytochrome P450-catalyzed reactions

Most cytochrome P450 (P450 or CYP)-catalyzed reactions are adequately described by classical Michaelis-Menten kinetic parameters (e.g., Km and Vmax), which are usually determined by a saturation profile of velocity of product formation versus substrate concentration. In turn, these parameters may be used to predict pharmacokinetics. However, some P450 enzymes exhibit atypical or non-Michaelis-Menten kinetics, due largely to substrate inhibition at higher concentrations of substrate. Although the mechanism of substrate inhibition is unknown, ignoring it and truncating the data can lead to erroneous estimates of kinetic parameters. In the present study, 13 P450 marker substrates were examined with 10 recombinant P450 proteins, and 6 were found, to varying degrees, to exhibit substrate inhibition. To understand the nature of the inhibition, a kinetic model was proposed (assuming that two binding sites exist on the enzyme) and used to fit the experimental data. The derived data indicated that 1) the K(I) values (substrate inhibition) were approximately 1.2- to 10-fold greater than the respective K(S) values; 2) both K(S) and K(I) values may be affected by the interaction of the two bound substrates within the enzyme, exhibited by a factor alpha (alpha = 5.1-23.3); and 3) enzyme activity was inhibited markedly (39-97%) at excess concentrations of the substrates (beta = 0.03-0.61). These findings suggest that substrates have access to both the inhibitory site and catalytic site simultaneously (K(I) > K(S)). Furthermore, the two sites, in the presence of substrate, can interact with each other. Therefore, the degree of inhibition of the enzyme is dependent on the concentration of the substrate (usually >K(I)) that sufficiently occupies the inhibitory site.

Enzyme kinetics of cytochrome P450-mediated reactions

The most common drug-drug interactions may be understood in terms of alterations of metabolism, associated primarily with changes in the activity of cytochrome P450 (CYP) enzymes. Kinetic parameters such as Km, Vmax, Ki and Ka, which describe metabolism-based drug interactions, are usually determined by appropriate kinetic models and may be used to predict the pharmacokinetic consequences of exposure to one or multiple drugs. According to classic Michaelis-Menten (M-M) kinetics, one binding site models can be employed to simply interpret inhibition (pure competitive, non-competitive and uncompetitive) or activation of the enzyme. However, some cytochromes P450, in particular CYP3A4, exhibit unusual kinetic characteristics. In this instance, the changes in apparent kinetic constants in the presence of inhibitor or activator or second substrate do not obey the rules of M-M kinetics, and the resulting kinetics are not straightforward and hamper mechanistic interpretation of the interaction in question. These unusual kinetics include substrate activation (autoactivation), substrate inhibition, partial inhibition, activation, differential kinetics and others. To address this problem, several kinetic models can be proposed, based upon the assumption that multiple substrate binding sites exist at the active site of a particular P450, and the resulting kinetic constants are, therefore, solved to adequately describe the observed interaction between multiple drugs. The following is an overview of some cytochrome P450-mediated classic and atypical enzyme kinetics, and the associated kinetic models. Applications of these kinetic models can provide some new insights into the mechanism of P450-mediated drug-drug interactions.

A kinetic model for the metabolic interaction of two substrates at the active site of cytochrome P450 3A4

In many cases, CYP3A4 exhibits unusual kinetic characteristics that result from the metabolism of multiple substrates that coexist at the active site. In the present study, we observed that alpha-naphthoflavone (alpha-NF) exhibited a differential effect on CYP3A4-mediated product formation as shown by an increase and decrease, respectively, of the carboxylic acid (P(2)) and omega-3-hydroxylated (P(1)) metabolites of losartan, while losartan was found to be an inhibitor of the formation of the 5,6-epoxide of alpha-NF. Thus, to address this problem, a kinetic model was developed on the assumption that CYP3A4 can accommodate two distinct and independent binding domains for the substrates within the active site, and the resulting velocity equations were employed to predict the kinetic parameters for all possible enzyme-substrate species. Our results indicate that the predicted values had a good fit with the experimental observations. Therefore, the kinetic constants can be used to adequately describe the nature of the metabolic interaction between the two substrates. Applications of the model provide some new insights into the mechanism of drug-drug interactions at the level of CYP3A4.

Major role of human liver microsomal cytochrome P450 2C9 (CYP2C9) in the oxidative metabolism of celecoxib, a novel cyclooxygenase-II inhibitor

In vitro studies were conducted to identify the cytochromes P450 (CYP) involved in the oxidative metabolism of celecoxib. The hydroxylation of celecoxib conformed to monophasic Michaelis-Menten kinetics (mean +/- S.D., n = 4 livers, K(m) = 3.8 +/- 0.95 microM, V(max) = 0.70 +/- 0.45 nmol/min/mg protein) in the presence of human liver microsomes, although substrate inhibition was significant at higher celecoxib concentrations. The treatment of a panel of human liver microsomal samples (n = 16 subjects) with antibodies against CYP2C9 and CYP3A4 inhibited the formation of hydroxy celecoxib by 72 to 92% and 0 to 27%, respectively. The presence of both antibodies in the incubation suppressed the activity by 90 to 94%. In addition, the formation of hydroxy celecoxib significantly correlated with CYP2C9-selective tolbutamide methyl hydroxylation (r = 0.92, P <. 001) and CYP3A-selective testosterone 6beta-hydroxylation (r = 0.55, P <.02). In contrast, correlation with activities selective for other forms of CYP was weak (r </= 0.46). Chemical inhibition studies showed that ketoconazole (selective for CYP3A4) and sulfaphenazole (selective for CYP2C9) inhibited the formation of hydroxy celecoxib in a concentration-dependent manner, whereas potent inhibitors selective for other forms of CYP did not show any significant effect over a range of 1 to 10 microM. In agreement, cDNA-expressed CYP2C9 catalyzed the formation of hydroxy celecoxib with an apparent K(m) value (microM) and a V(max) value (pmol/min/pmol recombinant CYP) of 5.9 and 21.7, whereas a higher K(m) value (18.2) and a lower V(max) value (1.42) were obtained with rCYP3A4. It is concluded that methyl hydroxylation of celecoxib is primarily catalyzed by human liver microsomal CYP2C9, although CYP3A4 also plays a role.

Mechanism-based inactivation of cytochrome P450 3A4 by L-754,394

Mechanism-based inactivation of human liver P450 3A4 by L-754,394, a Merck compound synthesized as a potential HIV protease inhibitor, was investigated using recombinant P450 3A4. Enzyme inactivation was characterized by a small partition ratio (3.4 or 4.3 +/- 0.4), i.e., the total number of metabolic events undergone by the inhibitor divided by the number of enzyme inactivating events, lack of reversibility upon extensive dialysis, no decrease in the characteristic 450-nm species relative to control, and covalent modification of the apoprotein. The major and minor products formed during the inactivation of P450 3A4 were the monohydroxylated and the dihydrodiol metabolites of L-754,394, respectively. L-754,394 that had been adducted to P450 3A4 was hydrolyzed under the conditions used for SDS-PAGE, Ni(2+) affinity chromatography, and proteolytic digestion. In addition, the modification was not stable to the acidic conditions of HPLC separation and CNBr digestion. The labile nature of the peptide adduct and the nonstoichiometric binding of the inactivating species to P450 3A4 precluded the direct identification of a covalently modified amino acid residue or the peptide to which it was attached. However, Tricine SDS-PAGE in combination with MALDI-TOF-MS and homology modeling, allowed I257-M317 to be tentatively identified as an active site peptide, while prior knowledge of the stability of N-, O-, and S-linked conjugates of activated furans implicates Glu307 as the active site amino acid that is labeled by L-754, 394.

Bioreactor systems in drug metabolism: synthesis of cytochrome P450-generated metabolites

In this communication, we report that suspension cultures of Sf21 insect cells, co-infected with baculovirus containing the cDNA for a single cytochrome P450 and NADPH-cytochrome P450 oxidoreductase, can be employed successfully as "bioreactors" for the synthesis of milligram quantities of cytochrome P450-generated metabolite(s). Three standard or probe substrates for the human P450s were chosen for the initial biosynthetic experiments: testosterone, diazepam, and diclofenac. Testosterone (100 microM, 2.88 mg/100 ml), added to a 100-ml CYP3A4 bioreactor, was converted to 6beta-hydroxytestosterone (2.3 mg) and 15beta-hydroxytestosterone (0.18 mg). Diazepam (100 microM, 2.9 mg/100 ml), added to a 100-ml CYP3A4 bioreactor, was converted to temazepam (1.1 mg), N-demethyldiazepam (0.35 mg), and oxazepam (0.15 mg). Diclofenac (100 microM, 3.18 mg/100 ml), added to a 100-ml CYP2C9 bioreactor, was converted to 4'-hydroxydiclofenac (2.6 mg). Since the goal for the development of the bioreactors was to provide a platform for both the production and subsequent purification of milligram quantities of P450-generated metabolite(s), a second 100-ml CYP2C9 bioreactor was used for the large-scale production and subsequent purification of 4'-hydroxydiclofenac. After 55 h of incubation, 7.95 mg of diclofenac was converted to 4.35 mg of 4'-hydroxydiclofenac, while 3.55 mg of unchanged diclofenac remained in the bioreactor. Using a simple preparative HPLC method, approximately 2.2 mg of 4'-hydroxydiclofenac and 1.9 mg of diclofenac were recovered from this experiment (28% yield). These results indicate clearly that suspension cultures of Sf21 insect cells coexpressing a cytochrome P450 and NADPH-cytochrome P450 oxidoreductase can be used effectively as bioreactors for the production and subsequent purification of milligram quantities of P450-derived metabolite(s).

Role of a potent inhibitory monoclonal antibody to cytochrome P-450 3A4 in assessment of human drug metabolism

Cytochrome P-450 (CYP) 3A4 is an inordinately important CYP enzyme that catalyzes the metabolism of a vast array of clinically used drugs. Microsomal proteins of Spodoptera frugiperda (Sf21) insect cells infected with recombinant baculoviruses encoding CYP3A4 cDNA were used to immunize mice and to develop a monoclonal antibody (mAb(3A4a)) specific to CYP3A4 through the use of hybridoma technology. The mAb is both a potent inhibitor and a strong binder of CYP3A4. One and 5 microl (0.5 and 2.5 microM IgG(2a)) of the mAb mouse ascites in 1-ml incubation containing 20 pmol of CYP3A4 strongly inhibited the testosterone 6beta-hydroxylation by 95 and 99%, respectively, and, to a lesser extent, cross-inhibited CYP3A5 and CYP3A7 activity. mAb(3A4a) exhibited no cross-reactivity with any of the other recombinant human CYP isoforms (CYP1A1, CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, and CYP2E1) in the course of CYP reaction phenotyping and Western immunoblot analyses. The potency of mAb-induced inhibition is insensitive to substrate concentration in human liver microsomes. Therefore, mAb(3A4a) was used to assess the quantitative role of CYP3A4/5 to the metabolism of testosterone and diazepam in five human liver microsomes. The results showed that CYP3A4 and CYP3A5 contribute >95% to both testosterone 6beta-hydroxylation and diazepam 3-hydroxylation and 52 to 73% to diazepam N-demethylation, respectively. In addition, mAb(3A4a) significantly inhibited testosterone 6beta-hydroxylase activity in rhesus monkey liver microsomes to a degree equal to that observed with CYP3A4 in human liver microsomes. By comparison, no inhibition of testosterone 6beta-hydroxylase activity was observed in the presence of dog, rat, and mouse liver microsomes. The selectivity of ketoconazole, a chemical inhibitor of CYP3A4, was probed with mAb(3A4a) and was shown to be highly concentration dependent in the diazepam N-demethylation by human liver microsomes. The results demonstrate that inhibitory and immunoblotting mAb(3A4a) can offer a precise and useful tool for quantitative identification of CYP3A4/5 in the metabolism of drugs in clinical use and drugs in development.

Sigmoidal kinetic model for two co-operative substrate-binding sites in a cytochrome P450 3A4 active site: an example of the metabolism of diazepam and its derivatives

Cytochrome P450 3A4 (CYP3A4) plays a prominent role in the metabolism of a vast array of drugs and xenobiotics and exhibits broad substrate specificities. Most cytochrome P450-mediated reactions follow simple Michaelis-Menten kinetics. These parameters are widely accepted to predict pharmacokinetic and pharmacodynamic consequences in vivo caused by exposure to one or multiple drugs. However, CYP3A4 in many cases exhibits allosteric (sigmoidal) characteristics that make the Michaelis constants difficult to estimate. In the present study, diazepam, temazepam and nordiazepam were employed as substrates of CYP3A4 to propose a kinetic model. The model hypothesized that CYP3A4 contains two substrate-binding sites in a single active site that are both distinct and co-operative, and the resulting velocity equation had a good fit with the sigmoidal kinetic observations. Therefore, four pairs of the kinetic estimates (KS1, kalpha, KS2, kbeta, KS3, kdelta, KS4 and kgamma) were resolved to interpret the features of binding affinity and catalytic ability of CYP3A4. Dissociation constants KS1 and KS2 for two single-substrate-bound enzyme molecules (SE and ES) were 3-50-fold greater than KS3 and KS4 for a two-substrate-bound enzyme (SES), while respective rate constants kdelta and kgamma were 3-218-fold greater than kalpha and kbeta, implying that access and binding of the first molecule to either site in an active pocket of CYP3A4 can enhance the binding affinity and reaction rate of the vacant site for the second substrate. Thus our results provide some new insights into the co-operative binding of two substrates in the inner portions of an allosteric CYP3A4 active site.

Studies on cytochrome P-450-mediated bioactivation of diclofenac in rats and in human hepatocytes: identification of glutathione conjugated metabolites

The nonsteroidal anti-inflammatory drug diclofenac causes a rare but potentially fatal hepatotoxicity that may be associated with the formation of reactive metabolites. In this study, three glutathione (GSH) adducts, namely 5-hydroxy-4-(glutathion-S-yl)diclofenac (M1), 4'-hydroxy-3'-(glutathion-S-yl)diclofenac (M2), and 5-hydroxy-6-(glutathion-S-yl)diclofenac (M3), were identified by liquid chromatography-tandem mass spectrometry analysis of bile from Sprague-Dawley rats injected i.p. with a single dose of diclofenac (200 mg/kg). These adducts presumably were formed via hepatic cytochrome P-450 (CYP)-catalyzed oxidation of diclofenac to reactive benzoquinone imines that were trapped by GSH conjugation. In support of this hypothesis, M1, M2, and M3 were generated from diclofenac in incubations with rat liver microsomes in the presence of NADPH and GSH. Increases in adduct formation were observed when incubations were performed with liver microsomes from phenobarbital- or dexamethasone-treated rats. Adduct formation was inhibited by polyclonal antibodies against CYP2B, CYP2C, and CYP3A (40-50% inhibition at 5 mg of IgG/nmol of CYP) but not by an antibody against CYP1A. Maximal inhibition was obtained when the three inhibitory antibodies were used in a cocktail fashion (70-80% inhibition at 2.5 mg of each IgG/nmol of CYP). These data suggest that diclofenac undergoes biotransformation to reactive metabolites in rats and that CYP isoforms of the 2B, 2C, and 3A subfamilies are involved in this bioactivation process. With respect to CYP2C isoforms, rat hepatic CYP2C7 and CYP2C11 were implicated as mediators of the bioactivation based on immunoinhibition studies using antibodies specific to CYP2C7 and CYP2C11. Screening for GSH adducts also was carried out in human hepatocyte cultures containing diclofenac, and M1, M2, and M3 again were detected. It is possible, therefore, that reactive benzoquinone imines may be formed in vivo in humans and contribute to diclofenac-mediated hepatic injury.

Electrospray ionization mass spectrometric analysis of intact cytochrome P450: identification of tienilic acid adducts to P450 2C9

A general scheme for the purification of baculovirus-expressed cytochrome P450s (P450s) from the crude insect cell pastes has been designed which renders the P450s suitable for analysis by high-performance liquid chromatography (HPLC) electrospray ionization mass spectrometry (ESI-MS). An HPLC/ESI-MS procedure has been developed to analyze small amounts of intact purified P450 (P450s cam-HT, 1A1, 1A2, 2A6, 2B1, 2C9, 2C9 C175R, 3A4, 3A4-HT) and rat NADPH cytochrome P450 reductase (P450 reductase). The experimentally determined and predicted (based on the amino acid sequences) molecular masses (MMs) of the various proteins had identical rank orders. For each individual protein, the difference between the experimentally determined (+/-SD, based on experiments performed on at least 3 different days) and predicted MMs ranged from 0.002 to 0.035%. Each experimentally determined MM had a standard deviation of less than 0.09% (based on the charge state distribution). Application of this HPLC/ESI-MS technique made the detection of the covalent modification to P450 2C9 following mechanism-based inactivation by tienilic acid possible. In the absence of glutathione, three P450 2C9 species were detected that produced ESI mass spectra corresponding to native P450 2C9 and both a monoadduct and a diadduct of tienilic acid to P450 2C9. In the presence of glutathione, only native P450 2C9 and the monoadduct were detected. Based on the observed mass shifts for the P450 2C9/tienilic acid adducts, a mechanism for the inactivation of P450 2C9 by tienilic acid is proposed.

Negative regulation of the rat glutathione S-transferase A2 gene by glucocorticoids involves a canonical glucocorticoid consensus sequence

Glucocorticoids (GCs) repress both basal and polyaromatic hydrocarbon-induced expression of the glutathione S-transferase Ya1 gene (gstA2) in isolated rat hepatocytes and rat liver in vivo. Transient transfection experiments with HepG2 cells were used to identify GC-responsive elements (GREs). With cotransfected GC receptor, chloramphenicol acetyltransferase (CAT) constructs containing a palindromic GRE (pGRE) and three GRE hexanucleotide half-sites between -1.6 and -1.1 kb of the 5'-flanking region of gstA2 were repressed >50% by GC when induced with polyaromatic hydrocarbon. This pGRE, if either mutated or deleted, significantly reduces GC responsiveness of the gene to 20-30%; no effect of GC was observed with CAT constructs containing -1.15 kb of the 5'-flanking region. The dexamethasone concentration dependence of the repression was consistent with involvement of the GC receptor and was antagonized by RU38486. Electrophoretic mobility shift assays demonstrated that pGRE formed a specific DNA/protein complex, which was prevented by the addition of excess unlabeled or mouse mammary tumor virus GRE but not by unrelated or mutated gstA2 GRE double-stranded oligonucleotides. This complex was supershifted by incubation of nuclear extracts containing GC receptor with anti-GC receptor globulins. Constructs containing multiple copies of pGRE sequence were either nonresponsive or positively responsive (three copies) to GC. Luciferase constructs containing -1.62 to -1.03 kb of the 5'-flanking region also were regulated positively by GC. Chimeric GC-peroxisome proliferator activated receptor activated the constructs that were positively responsive to GC but did not mediate the negative effect in constructs containing 1.6 kb of 5'-flanking region. We conclude that pGRE and half-site GREs of gstA2 participate in regulation of this gene; however, a second unidentified responsive element must exist between -1.03 and -0.164 kb, resulting in repression of gstA2 expression.

Genetic association between sensitivity to warfarin and expression of CYP2C9*3

Cytochrome P4502C9 (CYP2C9) is largely responsible for terminating the anticoagulant effect of racemic warfarin via hydroxylation of the pharmacologically more potent S-enantiomer to inactive metabolites. Mutations in the CYP2C9 gene result in the expression of three allelic variants, CYP2C9*1, CYP2C9*2 and CYP2C9*3. Both CYP2C9*2 and CYP2C9*3 exhibit altered catalytic properties in vitro relative to the wild-type enzyme. In the present study, a patient was genotyped who had proven unusually sensitive to warfarin therapy and could tolerate no more than 0.5 mg of the racemic drug/day. PCR-amplification of exons 3 and 7 of the CYP2C9 gene, followed by restriction digest or sequence analysis, showed that this individual was homozygous for CYP2C9*3. In addition, patient plasma warfarin enantiomer ratios and urinary 7-hydroxywarfarin enantiomer ratios were determined by chiral-phase high performance liquid chromotography in order to investigate whether either parameter might be of diagnostic value in place of a genotypic test. Control patients receiving 4-8 mg warfarin/day exhibited plasma S:R ratios of 0.50 +/- 0.25:1, whereas the patient on very low-dose warfarin exhibited an S:R ratio of 3.9:1. In contrast, the urinary 7-hydroxywarfarin S:R ratio of 4:1 showed the same stereoselectivity as that reported for control patients. Therefore, expression of CYP2C9*3 is associated with diminished clearance of S-warfarin and a dangerously exacerbated therapeutic response to normal doses of the racemic drug. Analysis of the plasma S:R warfarin ratio may serve as a useful alternative test to genotyping for this genetic defect.

Transcription regulation of rat glutathione S-transferase Ya subunit gene expression by chemopreventive agents

PURPOSE

To study the transcription regulation of rat glutathione S-transferase Ya (rGSTya) subunit gene expression by chemopreventive agents.

METHODS

The effects of chemopreventive agents; tamoxifen, genistein, oltipraz, indole-3-carbinol, and various isothiocyanates-sulforaphane, PMITC, PEITC, PBITC, and PPITC, on the transcriptional activation of rGSTya were investigated in cell culture. These were accomplished with a stable human hepatoma Hep G2 cell line transfected with a 1.6 kilobase (kb) 5'-flanking region of the rGSTya fused with the chloramphenicol acetyltransferase (CAT) reporter gene. Concentration-effect relationship and the kinetics of gene activation following treatments of the cells with different chemopreventive agents were carried out by quantitating CAT reporter protein using ELISA. Northern blot analysis of total RNA on the expression of CAT mRNA as well as potential transcription factors such as c-Jun, c-Fos, and LFR-1 were performed.

RESULTS

Treatment of the cells with increasing concentrations of different chemopreventive agents resulted in corresponding increases in the gene expression of CAT reporter protein. Kinetically, induction of CAT protein was seen as early as 3 hr and peaked at about 20 hr. Northern blot analysis revealed an increase in CAT mRNA transcripts and these mRNA inductions in general were in agreement with those quantitated by the production of CAT reporter protein. Induction of the transcription factor, c-Jun mRNA was observed with sulforaphane.

CONCLUSIONS

These results show that different chemopreventive agents transcriptionally activate rGSTya CAT in a time and dose-dependent fashion.

Transcriptional regulation of a rat liver glutathione S-transferase Ya subunit gene. Analysis of the antioxidant response element and its activation by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate

Using transfection and gel retardation assays, we have characterized further the antioxidant response element (ARE) found in the 5'-flanking region of the rat glutathione S-transferase Ya subunit gene. The ARE core sequence (5'-GTGACAAAGC-3') is sufficient for transcriptional activation of the Ya subunit gene by metabolizable planar aromatic compounds, phenolic antioxidants, and hydrogen peroxide. When the ARE sequence is ligated to a chloramphenicol acetyltransferase reporter gene and transfected into HepG2 cells, chloramphenicol acetyltransferase activity is modestly inducible by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Since the ARE is responsive to TPA and shows some sequence similarity to an AP-1-binding site (Jun/Fos recognition motif), we have explored whether members of the Jun/Fos family of transcription factors might bind to the ARE. Using in vitro synthesized Jun and Fos, binding to the ARE could not be detected, whereas Jun/Fos binding to a classical AP-1-binding site, a TPA response element (TRE) from the human collagenase gene, could be demonstrated by gel retardation assays. If the 2 A nucleotides underlined in the ARE core sequence (5'-GTGACAAAGC-3') are changed to TC, the ARE sequence (ARE-TRE) becomes a high-affinity AP-1-binding site and retains xenobiotic inducibility. Removal of the -GC- dinucleotide at the 3'-end of the ARE or the ARE-TRE eliminates xenobiotic inducibility. However, the ARE-TRE construct without the -GC- dinucleotide is still a high-affinity AP-1 site and responsive to TPA. Taken together, our data suggest that the ARE is not a high-affinity binding site for the Jun/Fos heterodimer. Functionally, however, an AP-1-binding site can resemble an ARE in its response to various xenobiotics if a 3'-GC- dinucleotide is present.

Cloning and expression of a cDNA for the human prostanoid IP receptor

A cDNA clone coding for a functional human prostanoid IP receptor has been isolated from a lung cDNA library. The human IP receptor consists of 386 amino acid residues with a predicted molecular mass of 40,961, and has the seven putative transmembrane domains characteristic of G-protein-coupled receptors. Challenge of Xenopus oocytes co-expressing the IP receptor and the cystic fibrosis transmembrane conductance regulator (cAMP-activated Cl- channel) with the stable prostacyclin analog iloprost resulted in specific inward Cl- currents, demonstrating that the cDNA encoded a functional IP prostanoid receptor coupled to elevation in cAMP. Radioreceptor binding studies using membranes prepared from mammalian COS cells transfected with the IP receptor cDNA showed that the rank order of potency for prostaglandins and prostaglandin analogs in competition for [3H]iloprost specific binding sites was as predicted for the IP receptor, with iloprost >> carbacyclin >> prostaglandin (PG) E2 > PGF 2 alpha = PGD2 = U46619. Northern blot analysis showed that IP mRNA was most abundantly expressed in kidney, with lesser amounts detected in lung and liver. In summary, we have cloned and expressed a cDNA for the human prostanoid IP receptor that is functionally coupled to a signaling pathway involving stimulation of intracellular cAMP production.

Cloning and expression of three isoforms of the human EP3 prostanoid receptor

Functional cDNA clones coding for three isoforms of the human prostaglandin E receptor EP3 subtype have been isolated from kidney and uterus cDNA libraries. The three isoforms, designated hEP3-I, hEP3-II and hEP3-III, have open reading frames corresponding to 390, 388 and 365 amino acids, respectively. They differ only in the length and amino acid composition of their carboxy-terminal regions, beginning at position 360. The human EP3 receptor has seven predicted transmembrane spanning domains and therefore belongs to the G-protein-coupled receptor family. The rank order of potency for prostaglandins and related analogs in competition for [3H]PGE2 specific binding to membranes prepared from transfected COS cells was comparable for all three isoforms, and as predicted for the EP3 receptor, with PGE2 = PGE1 >> PGF2 alpha = iloprost > PGD2 >> U46619. In addition, the EP3-selective agonist MB28767 was a potent competing ligand with an IC50 value of 0.3 nM, whereas the EP1-selective antagonist AH6909 gave IC50 values of 2-7 microM and the EP2-selective agonist butaprost was inactive. In summary, we have cloned three isoforms of the human EP3 receptor having comparable ligand binding properties.

Cloning and expression of a cDNA for the human prostanoid FP receptor

A cDNA clone coding for a functional human prostanoid FP receptor has been isolated from a uterus cDNA library. The human FP receptor consists of 359 amino acid residues with a predicted molecular mass of 40,060, and has the seven putative transmembrane domains characteristic of G-protein-coupled receptors. Challenge of Xenopus oocytes expressing the FP receptor with 10 nM of either prostaglandin (PG) F2 alpha or the selective FP-receptor agonist fluprostenol resulted in an elevation in intracellular Ca2+. Radioreceptor binding studies using membranes prepared from mammalian COS cells transfected with the FP receptor cDNA showed that the rank order of potency for prostaglandins and prostaglandin analogs in competition for [3H]PGF2 alpha specific binding sites was as predicted for the FP receptor, with PGF2 alpha approximately fluprostenol > PGD2 > PGE2 > U46619 > iloprost. In summary, we have cloned the human prostanoid FP receptor which is functionally coupled to the Ca2+ signalling pathway.

Enantioselective activation of the peroxisome proliferator-activated receptor

A cell-based transactivation assay was established using the mouse full-length peroxisome proliferator-activated receptor (PPAR) cDNA sequence and the positive peroxisome proliferator-responsive regulatory element (-578 to -553) of the rat acyl-CoA oxidase gene promoter. Activation of the reporter plasmid was dependent on co-transfection of the full-length PPAR cDNA, and the response was greatly stimulated, up to 100-fold, by peroxisome proliferators such as Wy-14,643 ([4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio] acetic acid), nafenopin (2-methyl-2[p-(1,2,3,4-tetrahydro-1-naphthyl)phenoxy]-propionic acid), and clofibric acid (2-([p]-chlorophenoxy)-2-methylpropionic acid). Activation of the reporter plasmid promoter by the full-length PPAR cDNA also occurred at peroxisomal proliferator concentrations 40 times lower than that required for similar stimulation by a glucocorticoid-PPAR chimeric receptor. By using the stereoisomers of MK-571 ((+-)-3-(((3-(2-(7-chloro-2-quinolinyl)ethenyl)-phenyl)((3- (dimethylamino)-3-oxopropyl)-thio)methyl)-thio)propanoic acid), a potent leukotriene D4 receptor antagonist, we could show enantioselective activation of PPAR. The use of this compound in mice results in peroxisome proliferation; however, nearly all of the peroxisome proliferating activity can be attributed to the S enantiomer. Our results show a similar enantiomeric discrimination in PPAR activation of the reporter plasmid promoter, where again most of the activity can be attributed to the S enantiomer. The equivalent activities of these stereoisomers both in vivo and in the PPAR transactivation assay strongly implicate PPAR as a major component of the peroxisome proliferating mechanism in rodents.

The antioxidant responsive element. Activation by oxidative stress and identification of the DNA consensus sequence required for functional activity

We have characterized further the antioxidant responsive element (ARE) identified in the 5'-flanking region of the rat glutathione S-transferase Ya subunit gene and the NAD(P)H:quinone reductase gene by mutational and deletion analyses. Our data suggest that the sequence, 5'-puGTGACNNNGC-3' 3'-pyCACTGNNNCG-5' where N is any nucleotide, represents the core sequence of the ARE required for transcriptional activation by phenolic antioxidants and metabolizable planar aromatic compounds (e.g. beta-naphthoflavone and 3-methylcholanthrene). We also have found that the ARE is responsive to hydrogen peroxide and phenolic antioxidants that undergo redox cycling. These latter data suggest that the ARE is responsive to reactive oxygen species and thus may represent part of a signal transduction pathway that allow eukaryotic cells to sense and respond to oxidative stress.

Protective activity of different hepatic cytosolic glutathione S-transferases against DNA-binding metabolites of aflatoxin B1

To evaluate the role of glutathione S-transferase (GST) isoenzymes in induced resistance of hepatocytes to aflatoxin B1 (AFB1), we compared DNA protective activities of different hepatic cytosol preparations and purified GSTs from normal rats, rats exposed to different polychlorinated biphenyls (PCBs), and rats with carcinogen-induced hepatocellular neoplasms, with cytosols or purified GSTs from mouse, rainbow trout, and human livers. These comparisons were performed in an in vitro assay for [3H]AFB1-DNA binding after activation by rat liver microsomes. Cytosol and S-hexylglutathione-affinity-purified GST preparations from livers of mice consistently had strong protective activity against AFB1-DNA binding. The majority of this activity was dependent on the presence of reduced glutathione (GSH) but some GSH-independent protection was observed in mouse hepatic cytosol, but not in purified GST preparations. We found that all of the GSH-dependent DNA-protective activity in mouse liver eluted as a single GST isoenzyme by hydroxyapatite chromatography. Preparations of cytosol and purified GSTs from normal rat liver, rainbow trout liver, and human liver had much less AFB1-specific DNA protective activity than GSTs found in mouse liver preparations. Cytosol from rats with carcinogen-generated liver neoplasms and livers induced with 3,3',4,4'-tetrachlorobiphenyl and 2,2',4,4',5,5'-hexachlorobiphenyl had more GST activity toward CDNB than cytosol from normal rat liver. When equivalent units of GST activity (CDNB) were compared, there was little difference observed between the DNA-protective activities of PCB-induced and normal rat liver cytosols, yet cytosol from rat liver neoplasms was more protective. Purified GST-P (7-7), the GST isoenzyme most induced in carcinogen-generated rat liver neoplasms, was not protective when added at protein concentrations found to be protective for total GSTs isolated from these neoplasms. These studies demonstrate that the resistance of mouse liver to AFB1 can be explained primarily by a single constitutive GST isoenzyme (YaYa or 4-4) with a relatively high activity toward DNA-binding metabolites of AFB1. GST isoenzymes with such high specific DNA protective activity against AFB1 metabolites were not evident in human, rat, or rainbow trout liver or in PCB-induced or neoplastic rat liver preparations.

Transcriptional regulation of the rat glutathione S-transferase Ya subunit gene. Characterization of a xenobiotic-responsive element controlling inducible expression by phenolic antioxidants

We have identified previously a xenobiotic-responsive element, which we termed the beta-naphthoflavone-responsive element, between nucleotide -722 and -682 in the 5'-flanking region of the rat glutathione S-transferase Ya subunit gene (Rushmore, T.H., King, R.G., Paulson, K.E., and Pickett, C.B. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 3826-3830). The beta-naphthoflavone-responsive element is responsible for part of the transcriptional activation of the Ya subunit gene by planar aromatic compounds but has a sequence distinct from the xenobiotic-responsive element found in multiple copies in the cytochrome P-450 IA1 gene and as a single copy in the Ya subunit gene. In the present study, we demonstrate that the beta-naphthoflavone-responsive element is required for the transcriptional activation of the Ya subunit gene by phenolic antioxidants such as t-butylhydroquinone through a mechanism that does not require functional Ah receptors. Furthermore, we present evidence that planar aromatic compounds must be metabolized before they transcriptionally activate the Ya subunit gene through the beta-naphthoflavone-responsive element. The transcriptional activation of the Ya subunit gene by planar aromatic compounds requires a functional Ah receptor. These data provide evidence that transcriptional activation of the glutathione S-transferase Ya subunit gene can be mediated by a novel xenobiotic-responsive element which is directly responsive to phenolic antioxidants such as t-butylhydroquinone. Hence we have named this new xenobiotic-responsive element the antioxidant-responsive element or ARE.

Regulation of glutathione S-transferase Ya subunit gene expression: identification of a unique xenobiotic-responsive element controlling inducible expression by planar aromatic compounds

We have identified a region in the 5' flanking sequence of the glutathione S-transferase (RX:glutathione R-transferase, EC 2.5.1.18) Ya subunit gene that contains a unique xenobiotic-responsive element (XRE). The regulatory region spans nucleotides -722 to -682 of the 5' flanking sequence and is responsible for part of the basal level as well as inducible expression of the Ya subunit gene by planar aromatic compounds such as beta-naphthoflavone (beta-NF) and 3-methyl-cholanthrene. The DNA sequence of this region (beta-NF-responsive element) is distinct from the DNA sequence of the XRE found in the cytochrome P-450 IA1 gene. In addition to the region containing the beta-NF-responsive element, two other regulatory regions of the Ya subunit gene have been identified. One region spans nucleotides -867 to -857 and has a DNA sequence with identity to the hepatocyte nuclear factor 1 recognition motif found in several liver-specific genes. The second region spans nucleotides -908 to -899 and contains a DNA sequence with identity to the XRE found in the cytochrome P-450 IA1 gene. The XRE sequence also contributes to part of the responsiveness of the Ya subunit gene to planar aromatic compounds. Our data suggest that regulation of gene expression by planar aromatic compounds can be mediated by a DNA sequence that is distinct from the XRE sequence.

Pathogenesis of skin and liver neoplasms in white suckers from industrially polluted areas in Lake Ontario

Increased prevalences of epidermal and hepatobiliary neoplasms in white suckers (Catostomu commersoni) and brown bullheads (Ictalurus nebulosus) in the Western region of Lake Ontario have been associated with industrial pollution, but the identity and causative role of environmental carcinogens have not yet been established. Most epidermal tumors of lip and body skin are benign focal proliferations that occur in fish from the polluted Hamilton region, and also in fish from less polluted sites in the Great Lakes. These skin tumors in white suckers do not have consistent alterations in cellular glutathione S-transferases (GST), suggesting that growth of skin tumors is not promoted by chemicals normally detoxified by GST. However, elevated levels of glutathione peroxidase (GPO) and glutathione reductase (GR) in skin papillomas are indicative of promotional peroxidative tissue injury, either caused directly by xenobiotics or indirectly by chemical-induced inflammation. Liver tumors in white suckers from Lake Ontario include preneoplastic, benign, and malignant populations of hepatocellular and biliary cells, all of which are more prevalent in fish from polluted sites. These liver tumors are consistently associated with chronic cholangiohepatitis and segmental cholangiofibrosis, but these conditions also occur in white suckers in non-industrial locations. Thus, the natural occurrence of biliary disease, not attributable to industrial pollution, may have some influence on the development of liver tumors. Some preneoplastic lesions and the majority of neoplastic hepatocellular and biliary lesions in white suckers have low levels of total GST, indicating that these liver neoplasms are not promoted by xenobiotics normally detoxified by hepatic GSTs.(ABSTRACT TRUNCATED AT 250 WORDS)

Prevention by free radical scavenger AD5 of prooxidant effects of choline deficiency

This study was designed to explore the possible preventive effects of a novel radicophile, N-p-methoxyphenylacetyl-dehydroalanine (AD5) and three other antioxidants, N,N'-diphenyl-p-phenylenediamine (DPPD), butylated hydroxyanisole (BHA) and a water-soluble analogue of vitamin E, trolox C, on the acute effects of the liver of feeding a choline-deficient (CD) diet. It has been suggested that some of the acute effects of a CD diet are related to free radicals, the generation or metabolism of which is disturbed in this acute dietary model. AD5 was found to be very effective in preventing nuclear lipid peroxidation, DNA damage and cell death induced by a CD diet but to have little effect on triglyceride accumulation ("fatty liver"). DPPD, BHA, and trolox C were ineffective. These results add strength to the hypothesis that oxygen free radicals might be an important component in the early events during carcinogenesis induced by feeding a CD diet.

Glutathione and enzymes related to free radical metabolism in liver of rats fed a choline-devoid low-methionine diet

Fischer F-344 male rats, fed a choline-devoid diet that leads to a highly reproducible sequence of biochemical and biological changes with an ultimate development of hepatocellular carcinoma, show elevated levels of glutathione in the liver at 3, 6 and 8 days. Several enzymes related to the metabolism of free radicals, including superoxide dismutase, catalase, glutathione peroxidase, glutathione S-transferase and DT-diaphorase show neither increased nor decreased activity as measured between 12 h and 8 days on the diet. Thus, of several known cellular components related to the possible scavenger of free radicals in the liver, only glutathione responded to the feeding of the CD diet. It is tentatively concluded that a decrease in the levels of possible scavengers for free radicals is not a major basis for the nuclear and mitochondrial lipid peroxidation seen early in rats fed a choline-devoid diet.

Purification and characterization of P-52 (glutathione S-transferase-P or 7-7) from normal liver and putative preneoplastic liver nodules

A previous study from our laboratory (L.C. Eriksson et al., Biochem. Biophys. Res. Commun., 117: 740-745, 1983) revealed that a cytosolic polypeptide of approximate Mr 21,000 (designated P-21) was markedly elevated in amount in hepatocyte nodules induced by six different regimens. The molecular weight of this polypeptide, subsequently revised to approximately 26,000, was redesignated P-26 and was identified (T.H. Rushmore et al., Biochem. Biophys. Res. Commun., 143: 98-103, 1987) as a subunit of a placental form of glutathione S-transferase (K. Sato et al., Gann 75: 199-202, 1984), also named glutathione S-transferase 7-7 (H. Jensson et al., FEBS Lett., 187: 115-120, 1985). We describe here a convenient method for purifying relatively large amounts of P-26 from hepatocyte nodules involving the sequential use of affinity chromatography on S-hexyl glutathione-Sepharose 4B, CM-Sephadex, and DEAE-Sephacel. Evidence is presented that P-26 exists as a dimer of approximate Mr 52,000 (P-52). Analyses by two-dimensional electrophoresis have indicated that the subunits of Mr 26,000 may consist of five separate charged isomers. Investigations using appropriate antisera and analysis by amino acid sequencing have provided additional confirmation that P-52 is probably identical to rat placental glutathione S-transferase. Antibodies to P-52 are proving to be useful as a marker of new cell populations that appear regularly during hepatocarcinogenesis.

Probable free radical effects on rat liver nuclei during early hepatocarcinogenesis with a choline-devoid low methionine diet

Fischer-344 rats fed a choline-devoid diet show lipid peroxidation in the liver nuclei, beginning at 1 day, reaching a peak at 3 days, and subsequently declining by 35 days. Lipid peroxidation in the mitochondria was seen first at 3 days, increased to a maximum at 28 days, and decreased after 35 days to undetectable values at 49 days. Lipid peroxidation was found in both nuclear and mitochondrial fractions both before and after stripping of their outer membranes. No microsomal lipid peroxidation could be detected at any time up to 63 days. The animals fed the same diet supplemented with choline showed no lipid peroxidation in any liver fraction. Animals given CCl4 showed the expected lipid peroxidation in the microsomes but not in the nuclear fraction. The administration of the free radical trapping agent, N-tert-butyl-alpha-phenylnitrone, prevented completely or almost so, microsomal lipid peroxidation induced by CCl4 and nuclear lipid peroxidation in the animals fed the choline-devoid, low methionine diet. The genesis of free radicals in the livers of rats fed a choline-devoid diet is considered as a likely hypothesis for the observed lipid peroxidation. The lipid peroxidation in turn is considered to be closely related to the induction of liver cell death and to the production of alterations in DNA. The DNA alterations coupled with regenerative liver cell proliferation suggest an attractive hypothesis for the initiation of hepatocarcinogenesis in rats fed a choline-devoid diet.

Mitogenic activity in platelet-poor plasma from rats with persistent liver nodules or liver cancer

Platelet-poor plasma (PPP) from F-344 rats with chemically-induced preneoplastic liver nodules or hepatocellular carcinoma stimulated S-phase DNA synthesis in monolayer cultures of normal rat hepatocytes. Similar mitogenic activity was detected in PPP 6 hrs to 1 week after partial hepatectomy (PH) or after necrotizing doses of CCl4 or diethylnitrosamine (DENA). Very little activity was found in PPP4 from control rats. The mitogenic activity in PPP from animals with nodules was non-dialyzable (greater than 14 kd) and bound to a heparin-sepharose affinity column. None of the mitogenic PPPs competed with [125I] epidermal growth factor (EGF) for binding sites on A431 cells or normal rat hepatocytes. These studies indicate that persistent proliferation of preneoplastic and neoplastic hepatocytes is associated with increased circulating levels of mitogenic hepatocyte growth factor.

Initiation of carcinogenesis by a dietary deficiency of choline in the absence of added carcinogens

The feeding for 10 or 11 weeks of young male Fischer-344 rats, a diet devoid of choline and low in methionine, leads to the appearance of gamma-glutamyltransferase-positive foci of altered hepatocytes in the liver and to the induction of initiated resistant hepatocytes. The latter are known to contain the primary precursor cells for the ultimate development of hepatocellular carcinoma. This initiation of carcinogenesis with the choline-devoid diet is prevented by added choline. These observations indicate that a dietary deficiency may, by itself, without known contaminating or added carcinogens, initiate the carcinogenic process.

Inhibition of hepatocarcinogenic responses to 1,2-dimethylhydrazine by diallyl sulfide, a component of garlic oil

The mechanisms by which diallyl sulfide (DAS), a component of garlic oil, inhibits hepatocarcinogenicity of 1,2-dimethylhydrazine (DMH) were examined in male Fischer 344 rats. Rats were subjected to partial hepatectomy to stimulate hepatocellular proliferation required for initiation by DMH (50-200 mg/kg i.p.) given 12 h later. Initiation was assessed by the numbers of foci and nodules of hepatocytes that were positive for gamma-glutamyl transpeptidase (gamma-GT) or glutathione-S-transferase-P (GST-P) after 6 weeks promotion by orotic acid (1% in semi-purified diet). DAS at doses above 50 mg/kg (by gavage) administered 1 h before DMH (50 mg/kg) partially reduced the numbers of gamma-GT and GST-P-positive foci. By comparison, all doses of DAS (25-100 mg/kg) completely prevented liver necrosis by DMH (200 mg/kg). DAS substantially reduced macromolecular binding of [14C]DMH in cultured liver cells, but had no effect on their levels of glutathione-S-transferase, glutathione reductase or glutathione peroxidase at 18 h. These findings suggest that low dosages of DAS which reduce DMH binding appear more likely to inhibit hepatocarcinogenicity by reducing the promoting influences of post-necrotic regeneration than by preventing initiation.

Modulation of calcium by the carcinogenic process in the liver induced by a choline-deficient diet

A diet devoid of choline and low in methionine (CD), without any added carcinogen, has been shown to induce 100% preneoplastic nodules and more than 50% cancer in the rat liver. Attempts to understand the mechanism by which a CD diet induces liver cell cancer revealed that like chemical carcinogens, a CD diet also appears to cause alterations in DNA, perhaps mediated by free radicals. Indeed, a CD diet induces nuclear lipid peroxidation prior to the changes in DNA. The CD diet induced DNA alterations coupled with continuing liver cell proliferation may account for the induction of initiated hepatocytes by the CD diet. To gain insight into the nature of free radicals generated by the CD diet, experiments were designed to determine whether agents that modulate free radical effects influence the CD diet induced changes in the liver. We investigated the effect of Ca2+ in the modulation of CD diet induced alterations in the liver. The results show that extra Ca2+ when added to the CD diet prevented some of the early changes due to choline deficiency, such as nuclear lipid peroxidation and DNA damage, but had little or no effect on the triglyceride accumulation in the liver. Also, the same CD diet with extra Ca2+, when used as a promoter after initiation by diethylnitrosamine, decreased the number and size of early putative preneoplastic foci and nodules.

Identification of a characteristic cytosolic polypeptide of rat preneoplastic hepatocyte nodules as placental glutathione S-transferase

Evidence is presented that a distinctive type of glutathione-S-transferase (GSHTase-P), and a cytosolic polypeptide of Mr 52,000 (P-52), each appearing in greatly increased amounts in hepatocyte nodules during liver carcinogenesis in the rat, are so far indistinguishable. The probable identity of the two polypeptides was established with the use of SDS polyacrylamide gel electrophoresis and Western blot techniques with purified GSHTase-P and P-52 and their respective antibodies and by comparison of the sequence of the first 26 N-terminal amino acids. Since the enzyme and the polypeptide are each considered to be the best available early markers for hepatocyte nodules, as putative precancerous lesions, their probable identity makes them attractive cellular components for in depth studies on their transcriptional and translational regulation and their use in new approaches to the sequential analysis of liver carcinogenesis.

A choline-devoid diet, carcinogenic in the rat, induces DNA damage and repair

A diet deficient in choline when fed to rats for as few as 3 days resulted in liver DNA damage. The damage could be monitored as alkali-labile lesions using alkaline-sucrose gradients as well as alkaline elution technique. The DNA damage disappeared when the choline-deficient diet was replaced by a choline-supplemented diet suggesting the alkali-labile lesions were repairable. The DNA damage was detected at a time when no liver cell death was detectable. The induction of DNA damage in a proliferating liver by the choline-deficient diet may be an early important event leading to initiation of liver carcinogenesis.

Influences of glutathione status on different cytocidal responses of monolayer rat hepatocytes exposed to aflatoxin B1 or acetaminophen

In short-term primary monolayer cultures of rat hepatocytes, aflatoxin B1 (AFB1) causes a characteristic prelethal cytomorphological response in which peripheral attached cytoplasm contracts segmentally to form finger-like blebs. This response precedes lethal injury as detected by release of lactate dehydrogenase (LDH) into culture medium. We compared the influences of various modifiers of cellular glutathione (GSH) status on cytocidal responses of Fischer 344 rats hepatocytes exposed to AFB1 or acetaminophen (AAP), a hepatotoxin which does not produce segmental cytoplasmic contraction. N-Acetylcysteine (4 mM) reduced the degree of LDH release by AAP (4 to 16 mM) but was not protective against cell killing by AFB1, although it slightly reduced the percentage of hepatocytes with segmental cytoplasmic contraction at 6 hr. BCNU (1,3-bis(2-chloroethyl)-1-nitrosourea) at 40 microM markedly inhibited glutathione reductase and also strongly potentiated cell killing by AAP but did not significantly influence segmental cytoplasmic contraction or LDH release in response to AFB1. Diethylmaleate (40 to 160 microM), a depletor of hepatocellular GSH, and buthionine-D,L-sulfoximine (4 mM), an inhibitor of GSH synthesis, each did not alter hepatocyte killing by AFB1 but were strong potentiators of toxicity of AAP. AAP inhibited glutathione reductase but AFB1 did not. Total GSH concentrations at 6 and 18 hr were reduced by AAP and to a lesser extent by AFB1 in comparison with control cultures. These findings demonstrate that, in contrast to AAP toxicity, the characteristic mode of hepatocyte killing by AFB1 in monolayer cultures is substantially independent of induced alterations in GSH. These results indicate that GSH-dependent detoxification mechanisms do not play a major role in removing necrogenic metabolites of AFB1 in Fischer 344 rat hepatocytes. They further suggest that prelethal responses of AFB1-injured hepatocytes are not affected by GSH-dependent cytoprotective mechanisms.

Inhibition of RNA synthesis by benzene metabolites and their covalent binding to DNA in rabbit bone marrow mitochondria in vitro

Mitoplasts from rabbit bone marrow cells in vitro incubated with benzene metabolites showed a concentration-dependent inhibition of mitochondrial RNA synthesis. The 50% molar inhibitory concentration for each metabolite was determined to be p-benzoquinone, 2 X 10(-6); phenol, 2.5 X 10(-5); hydroquinone, 5 X 10(-5); catechol, 2 X 10(-3); benzene, 1.6 X 10(-2). Activated metabolites of radiolabeled benzene covalently bound to mitochondrial DNA in vitro. Labeled deoxynucleosides, enzymatically produced from DNA isolated from mitoplasts prelabeled in DNA with [3H]dGTP and exposed to [14C]benzene in vitro, were chromatographed on a Sephadex LH-20 column to obtain deoxynucleoside-adducts. The elution profiles indicated that mtDNA contained seven deoxyguanosine-adducts. A similar experiment in which the mtDNA was prelabeled with [3H]dATP indicated that two deoxyadenosine-adducts of mtDNA were formed from benzene in vitro. Tentative identification of several deoxyguanosine-adducts indicates that p-benzoquinone, hydroquinone, phenol, and 1,2,4-benzenetriol produced from benzene form adducts with guanine.

Rapid lipid peroxidation in the nuclear fraction of rat liver induced by a diet deficient in choline and methionine

A diet deficient in choline and methionine, known to produce hepatocellular carcinoma in the absence of any added chemical carcinogen, induced lipid peroxidation in the nuclear fraction of the liver when fed to male Fischer 344 rats. This lipid peroxidation was detected within 1 day of feeding the diet by the appearance of diene conjugates and increased progressively up to 3 days. It was prevented completely by the addition of choline chloride to the diet. The close proximity of DNA may make it a possible target for attack by free radicals.