The Integrated Use of Alternative Methods in Toxicological Risk Evaluation - ECVAM Integrated Testing Strategies Task Force Report 1

The ECVAM Task Force on Integrated Testing Strategies was established in December 1996, with the remit of assessing the current status of integrated toxicity testing, and of making proposals regarding the design and implementation of integrated testing strategies. The first step in an integrated testing strategy is usually to determine the chemical functionality of a substance, on the basis of its structure and physicochemical properties. The biokinetic and dynamic behaviours of the chemical in various in vitro systems are then assessed. The various elements are then integrated, in either a parallel or a stepwise fashion, to make predictions of the local or systemic toxicity of the chemical of interest. In this report, a generic scheme for local/systemic toxicity, and a specific scheme for target organ toxicity, are proposed. The scope and limitations of the approaches are discussed. The task force hopes that its proposals will stimulate a discussion on the feasibility of this type of approach and it welcomes any feedback. It is planned that the discussion points will be elaborated in a second task force report.

In vitro neuroendocrine effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the AhR-expressing hypothalamic rat GnV-3 cell line

The aryl hydrocarbon receptor (AhR) is involved in a wide variety of biological and toxicological responses, including neuroendocrine signaling. Due to the complexity of neuroendocrine pathways in e.g. the hypothalamus and pituitary, there are limited in vitro models available despite the strong demand for such systems to study and predict neuroendocrine effects of chemicals. In this study, the applicability of the AhR-expressing rat hypothalamic GnV-3 cell line was investigated as a novel model to screen for neuroendocrine effects of AhR ligands using 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as reference compound. The qRT-PCR analyses demonstrated the presence of several sets of neurotransmitter receptors in the GnV-3 cells. TCDD (10nM) altered neurotransmitter signaling by up-regulation of glutamate (Grik2), gamma-amino butyric acid (Gabra2) and serotonin (Ht2C) receptor mRNA levels. However, no significant changes in basal and serotonin-evoked intracellular Ca(2+) concentration ([Ca(2+)]i) or serotonin release were observed. On the other hand, TCDD de-regulated period circadian protein homolog 1 (Per1) and gonadotropin releasing hormone (Gnrh) mRNA levels within a 24-h time period. Both Per1 and Gnrh genes displayed a similar mRNA expression pattern in GnV-3 cells. Moreover, the involvement of AhR in TCDD-induced alteration of Neuropeptide Y (Npy) gene expression was found and confirmed by using siRNA targeted against Ahr in GnV-3 cells. Overall, the combined results demonstrate that GnV-3 cells may be a suitable model to predict some mechanisms of action and effects of AhR ligands in the hypothalamus.

Phenobarbital pretreatment in vivo and in vitro and the effect of hepatotoxicity of d-galactosamine in rat hepatocytes in culture

Galactosamine (GalN) is a known hepatotoxic compound, acting by depletion of uracil nucleotides. The relation between an active cytochrome P-450 system (CYP) and the hepatotoxicity of GalN was studied in rat hepatocytes that were pretreated with phenobarbital (PB) in vivo or in vitro. A 24-hr in vitro pretreatment of cultured hepatocytes with PB resulted in a significant decrease in GalN toxicity as measured by lactate dehydrogenase (LDH) leakage. Furthermore, GalN treatment resulted in an increase in the activity of the PB-induced forms of CYP (namely CYP 2B1/2) as measured by 7-pentoxyresorufin O-depentylase (PROD) activity. This increase was not found after GalN treatment of microsomes. GalN had no effect on the concentration of the apoenzymes. GalN administration to hepatocytes of in vivo PB-pretreated rats resulted in a similar effect of GalN on the activity of the CYP enzymes but PB in vivo had no effect on GalN toxicity. These results suggest that GalN treatment may result in a significant increase in the specific activity of CYP 2B1/2 enzymes (PROD), without an obvious increase in the amount of PB-induced apoenzymes. This phenomenon was measurable only in intact cells. No direct relation is assumed between the activity of the CYP apoenzymes and the decrease in GalN toxicity after PB treatment. The toxicity of Galn was inhibited by PB treatment in vitro.

An alternative approach for the safety evaluation of new and existing chemicals, an exercise in integrated testing

Various in vitro and in silico methods without animals were applied to 10 substances listed on ELINCS with a complete VIIA base-set available at NOTOX. The hazard assessment for these substances was performed on basis of available non-animal data, QSAR, PBBK-modelling and additional, new in vitro testing was applied. Based on these data predictions on fish toxicity, acute toxicity, skin- and eye-irritation, sensitisation, and toxicity after repeated dosing were made. The predictions were compared with the outcome of the in vivo tests. Nine out of ten predictions on fish LC(50) proved to be correct. For skin- and eye-irritation 70% was predicted correctly. Sensitisation was predicted correctly for 7 out of 10 substances, but three false negatives were found. Acute oral toxicity (LD(50)) and repeated dose toxicity were less successful (5 out of 10 and 2 out of 10 correct predictions, respectively); application of the PBBK model proved successful. Acute dermal toxicity was predicted correctly in 9 out of 10 cases. In general an over-estimation of systemic toxicity was found, which can be explained by an over-prediction of cytotoxicity and worst case assumptions on absorption and binding to (plasma) proteins. This integrated approach leads to a 38% reduction of laboratory animals.

The integration of data on physico-chemical properties, in vitro-derived toxicity data and physiologically based kinetic and dynamic as modelling a tool in hazard and risk assessment. A commentary

Toxicity of a compound for an organism is dependent on the route of exposure, the amount (or concentration), the way in which the compound is taken up, distributes and is eliminated from the organism (ADME, kinetics) and the intrinsic properties (reactivity; mode of action, dynamics) of the compound towards the organism. These three elements: exposure, kinetics and dynamics form the basis of hazard and risk evaluations. Developments in our knowledge of the way in which physico-chemical properties of chemicals (on the one side) and physiological processes in the organism (on the other side) determine a compound's toxicity have greatly increased our understanding of toxicological processes and our ability to interpret experimental results. This has now resulted in the development of model systems in which the above-mentioned processes can be described mathematically. Biokinetic modelling is currently of great interest, but the further development of toxicodynamic modelling is equally important. The combination of both allows the estimation of a compound's critical amount/concentration on the critical site of action, which ideally would be the basis for hazard and risk assessments. In vitro systems have been extremely useful in studying the molecular basis of a chemical's biological activity, including its mechanism(s) of toxic action. Other achievements include the prediction of biological reactivity on the basis of a compound's physico-chemical properties and the construction of quantitative structure-activity relationships (QSARs). However, for the incorporation of in vitro-derived data as well as the results of QSARs, kinetic modelling is indispensable. Thus, biokinetic and toxicodynamic modelling are important (if not crucial) tools in toxicological research and there are increasing opportunities to incorporate the results of this work in hazard and risk assessments. Their implementation will allow a much more scientifically-based and a better structured risk assessment, which will be to a much lesser extent relying on animal experimentation.

Methods of in vitro toxicology

In vitro methods are common and widely used for screening and ranking chemicals, and have also been taken into account sporadically for risk assessment purposes in the case of food additives. However, the range of food-associated compounds amenable to in vitro toxicology is considered much broader, comprising not only natural ingredients, including those from food preparation, but also compounds formed endogenously after exposure, permissible/authorised chemicals including additives, residues, supplements, chemicals from processing and packaging and contaminants. A major promise of in vitro systems is to obtain mechanism-derived information that is considered pivotal for adequate risk assessment. This paper critically reviews the entire process of risk assessment by in vitro toxicology, encompassing ongoing and future developments, with major emphasis on cytotoxicity, cellular responses, toxicokinetics, modelling, metabolism, cancer-related endpoints, developmental toxicity, prediction of allergenicity, and finally, development and application of biomarkers. It describes in depth the use of in vitro methods in strategies for characterising and predicting hazards to the human. Major weaknesses and strengths of these assay systems are addressed, together with some key issues concerning major research priorities to improve hazard identification and characterisation of food-associated chemicals.

Toxicodynamic modelling and the interpretation of in vitro toxicity data

The results of in vitro toxicity experiments are not easily extrapolated to 'toxicological risk' for an intact organism. One of the most obvious differences between the situation in vitro and in vivo is the absence of the processes of absorption, distribution, metabolism and excretion that govern the exposure of the target tissues of the organism in vivo. The development of biokinetic models is aimed at estimating the relevant target tissue concentration of a compound. In our study, biokinetic models were constructed, where possible, solely on the basis of in vitro derived parameters for biotransformation as well as on partition coefficients determined or calculated from physicochemical structures. Another requirement is the existence of appropriate in vitro biological systems for the measurement of relevant effects. This requires a thorough knowledge of the possible mechanisms of toxic action, and of the physiology of the target organs. When these prerequisites are met (i.e. when the appropriate parameters can be quantified in a non-animal system), then an estimate of the dynamics in vitro can be made (e.g. as a critical active concentration). This will then result in a model describing a compound's dynamics. Eventually, the result of biokinetic and toxicodynamic models will need to be integrated in a compound's hazard and/or risk evaluation. A study carried out in the ECITTS programme showed promising results for the estimation of the acute and chronic systemic toxicity of a number of neurotoxic compounds.

13th meeting of the Scientific Group on Methodologies for the Safety Evaluation of Chemicals (SGOMSEC): alternative testing methodologies and conceptual issues

Substantial world-wide resources are being committed to develop improved toxicological testing methods that will contribute to better protection of human health and the environment. The development of new methods is intrinsically driven by new knowledge emanating from fundamental research in toxicology, carcinogenesis, molecular biology, biochemistry, computer sciences, and a host of other disciplines. Critical evaluations and strong scientific consensus are essential to facilitate adoption of alternative methods for use in the safety assessment of drugs, chemicals, and other environmental factors. Recommendations to hasten the development of new alternative methods included increasing emphasis on the development of mechanism-based methods, increasing fundamental toxicological research, increasing training on the use of alternative methods, integrating accepted alternative methods into toxicity assessment, internationally harmonizating chemical toxicity classification schemes, and increasing international cooperation to develop, validate, and gain acceptance of alternative methods.

Solid phase microextraction as a tool to determine membrane/water partition coefficients and bioavailable concentrations in in vitro systems

Solid phase microextraction (SPME) is an extraction technique that uses a polymer-coated fiber as the extraction device. After extraction, the compound of interest can be desorbed from the fiber and subsequently analyzed by GC or HPLC. One of the properties of SPME is that only the freely dissolved fraction of a chemical is available for partitioning to the extraction device. The method can be applied in a way that small amounts are extracted from the sample, which allows negligible depletion extraction. These two properties make SPME devices particularly suitable for measurements of free concentrations. In toxicological studies the free concentration is considered to be a more relevant parameter, concerning toxic effects, than the nominal concentration that is used most frequently. In the current study, the usefulness of this method to measure phospholipid/water partition coefficients and free concentrations in three different in vitro test systems (rat hepatocytes in primary culture, 9000 g and 100,000 g homogenate fractions of rainbow trout liver) was demonstrated. Results show separate relationships between phospholipid/water and n-octanol/water partition coefficients for a set of polar and nonpolar organic chemicals, respectively. These observations suggest that phospholipid/water partition coefficients may be a more suitable parameter in modeling the kinetic behavior of organic chemicals. Additionally, differences between the nominal and the actual free concentration in in vitro systems are more pronounced for more hydrophobic compounds, as was expected based on theoretical considerations. To our knowledge, the approach presented here is the first analytical method to measure toxicologically relevant concentrations in in vitro test systems in a fast and efficient way.

Simulation of lindane kinetics in rats

The kinetics of lindane were modelled in the male rat with a physiologically-based pharmacokinetic (PB-PK) model. The model was parameterized by using reference physiological parameter values and partition coefficients that were reported earlier in the literature. First order biotransformation and gastro-intestinal absorption constants for lindane were obtained by visually fitting the model to literature data on lindane disposition in vivo after a single oral dose. The model was validated by simulating the disposition of lindane in vivo after single intraperitoneal and chronic oral dosage and comparing simulated with experimental results. It was concluded that the present model can adequately simulate most of the reported data on lindane kinetics.

Simulation of toluene kinetics in the rat by a physiologically based pharmacokinetic model with application of biotransformation parameters derived independently in vitro and in vivo

The biokinetic behavior of toluene was modeled in the rat with a physiologically based pharmacokinetic (PB-PK) model. The model was parameterized by using reference physiological parameter values and partition coefficients that were reported earlier from in vitro studies. Biotransformation parameters for toluene, reported from two in vivo and six in vitro studies, were subsequently substituted in the model while keeping all other model parameters constant. Simulations of toluene kinetics, based on these eight biotransformation parameter sets, were compared with empirical data reported on toluene uptake in blood and/or brain tissue after inhalation exposure. It was observed that most empirical data on toluene blood concentrations were adequately predicted by the model for almost each of the eight biotransformation parameter sets. It was also observed that differences between model predictions, based on either in vivo- or in vitro-derived biotransformation parameters, were generally small. It is concluded that the results from most in vitro studies on toluene biotransformation can be applied successfully to predict the kinetics of toluene in vivo. It is also concluded that the brain-blood partition coefficient may be at least as important for the outcome of the model as the biotransformation parameters are. These results support earlier reported findings in the literature on application of in vitro techniques to derive parameters for PB-PK models.

Interactive effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin and retinoids on proliferation and differentiation in cultured human keratinocytes: quantification of cross-linked envelope formation

Dioxins are potent inducers of chloracne in humans. This skin aberration can be interpreted as an altered differentiation pattern of acinar sebaceous base cells and a change in the rate of terminal differentiation of the keratinocytes. We measured this rate induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in primary cultures of human keratinocytes. As parameters for differentiation, we quantified the 35S-methionine incorporation into cross-linked envelopes (revealing the total CLE biomass), as well as the number of microscopically visible CLEs. It was shown that TCDD is a very potent inducer of both CLE biomass and number with a half-maximal effect concentration (EC50) of 1.4 nM. CLE biomass was maximally increased 10-fold and the number of cells in culture producing a CLE was increased from 15% in control cultures to maximally 75% of the cells in TCDD-treated cultures. Both effects were Ca(2+)-dependent and increased with elevated cell density, being optimal in post-confluent cultures. Retinoic acid dose-dependently decreased the effect of 10(-8) M TCDD, 10(-6) M having a nearly complete antagonistic action. This interaction of retinoic acid with TCDD-induced differentiation was non-competitive. Retinol was equally potent as an antagonist of the TCDD-induced elevation of CLE formation as compared with retinoic acid. Retinyl palmitate and etretinate were not very effective as TCDD antagonists. Supplementation of hydrocortisone suppressed the TCDD-induced keratinocyte differentiation. It was concluded that CLE biomass quantification provides a reliable and sensitive parameter for keratinocyte differentiation. In this in vitro system it is shown that TCDD strongly induces a switch from proliferation to terminal differentiation and that this effect can be antagonized effectively by retinoic acid and retinol.

Role of thiol homeostasis and adenine nucleotide metabolism in the protective effects of fructose in quinone-induced cytotoxicity in rat hepatocytes

Freshly-isolated rat hepatocytes were exposed in glucose (15 mM) or fructose (5 mM) medium to menadione (2-methyl-1,4-naphthoquinone) (85 microM) or 1,4-naphthoquinone (NQ) (50 microM). Menadione and NQ are closely related quinones and have an approximately equal potential to induce redox cycling. However, NQ has a higher potential to arylate and is more toxic than menadione. During 2 hr of incubation, cell viability, thiol status, adenine nucleotide level and lactate production were determined. LDH-leakage was used as a measure of cell viability. In glucose medium, exposure of hepatocytes to menadione or NQ resulted in a faster excretion rate of oxidized glutathione as compared to those cells in fructose medium. As a result, quinone-exposed hepatocytes in fructose medium retained higher amounts of oxidized glutathione. Menadione-exposed hepatocytes in fructose medium exhibited a diminished rate of transthiolation of protein thiols with oxidized glutathione as compared to those cells in glucose medium. The adenine nucleotide level of hepatocytes in glucose medium was markedly higher than in fructose medium. This was caused by an ATP decrease in hepatocytes in fructose medium resulting in a low energy charge (E.C.) (0.6) as compared to hepatocytes in glucose medium (0.9). Only menadione caused a decrease in the E.C. in glucose medium while NQ caused a decrease of all three adenine nucleotides. In fructose medium, quinone-exposed hepatocytes showed no change in their adenine nucleotides as compared to control cells. Despite the higher oxidized glutathione content and the lower ATP level of NQ-exposed hepatocytes in fructose medium, they had a better viability than those cells in glucose medium. From our results we conclude that a high ATP content is not always beneficial for cell survival.

Effects of clofibric and beclobric acid in rat and monkey hepatocyte primary culture: influence on peroxisomal and mitochondrial beta-oxidation and the activity of catalase, glutathione S-transferase and glutathione peroxidase

The effect of hypolipidaemic compounds on peroxisomal fatty acid beta-oxidation and on peroxisome morphology in the liver differs widely between rodent and primate species. We studied the relative importance of peroxisomal and mitochondrial beta-oxidation of palmitate in primary cultures of hepatocytes isolated from rat and monkey liver in the absence or presence of clofibric acid or beclobric acid. It was demonstrated that it is possible to differentiate between peroxisomal and mitochondrial beta-oxidation activities in intact cells. Overall beta-oxidation of palmitate was ca. 30% higher in rat hepatocytes than in monkey liver cells. In both monkey and rat cell cultures the mitochondrial component was over 90% of the total palmitate beta-oxidation. In rat hepatocyte culture clofibric acid and beclobric acid caused a 5- to 8-fold stimulation of peroxisomal beta-oxidation, while in monkey cells this activity was not significantly increased. However, in cells derived from both species mitochondrial palmitate beta-oxidation was increased (rat 2.5-fold; monkey 1.5-fold). These results indicate that the species differences in the increase in peroxisomal fatty acid oxidation are not a result of an inability to metabolize fatty acids in rat liver cell mitochondria. A comparison of the activity of enzymes involved in the detoxification of hydrogen peroxide showed that catalase and glutathione-S-transferase activity is 2.9-fold higher in monkey hepatocytes than in rat liver cells, while glutathione peroxidase activity was 1.6-fold higher in rat cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of the protoporphyrinogen oxidase-inhibiting herbicide fomesafen on liver uroporphyrin and heptacarboxylic porphyrin in two mouse strains

The effect of the protoporphyrinogen oxidase-inhibiting herbicide fomesafen on liver porphyrin accumulation was studied in long-term high-dose experiments. Fomesafen caused liver accumulation of uroporphyrin and heptacarboxylic porphyrin when fed at 0.25% in the diet to male ICR mice for 5 months (fomesafen-treated mice: 52 nmol uroporphyrin, 21 nmol heptacarboxylic porphyrin/g liver; control mice: traces of uroporphyrin, heptacarboxylic porphyrin not detected). Uroporphyrinogen decarboxylase activity was depressed to about 25% of control values. Iron treatment accelerated the development of this porphyria cutanea tarda-like experimental porphyria both in ICR and C57B1/6J mice. In contrast to other uroporphyrinogen decarboxylase inhibitors, fomesafen treatment did not increase the cytochrome P450IA-related activities and the amount of P450IA2 protein was shown to be significantly decreased by Western immunoblotting. Thus, fomesafen is a unique chemical that inhibits both the oxidation of protoporphyrinogen as well as the conversion of uroporphyrinogen to coproporphyrinogen. However, the accumulation of highly carboxylated porphyrins is evident only after prolonged treatment with high doses of the herbicide.

Interindividual variation in biotransformation and cytotoxicity of bromobenzene as determined in primary hepatocyte cultures derived from monkey and human liver

1. Bromobenzene-evoked hepatotoxicity resulting from cytochrome P450-mediated epoxidation has been studied extensively in rodents in vivo and in rodent hepatocytes. In this paper we present data concerning the formation of bromphenols, glutathione (GSH) depletion and cytotoxicity observed in primate hepatocytes in primary culture after exposure to bromobenzene (BrB). 2. After pre-incubation for 2 or 24 h, hepatocytes were exposed to BrB in concentrations up to 2 mM for 4 or 24 h. 3. In both human and cynomolgus monkey hepatocytes BrB cytotoxicity and GSH depletion were found after exposure to 2 mM BrB. The degree of the observed effects was not influenced by the duration of pre-incubation and/or exposure periods. 4. Major inter-individual differences were observed, which could not be attributed to differences in cytochrome P450-mediated bioactivation rates. This suggests that the variation in individual susceptibility to BrB may be related to inter-individual differences in the activity of de-activating (metabolic) pathways. 5. The study of the background of these inter-individual differences may contribute to a more complete understanding of the factors ruling sensitivity to BrB or related chemicals.

Applicability of cultured hepatocytes derived from goat, sheep and cattle in comparative drug metabolism studies

1. Using trimethoprim (TMP), scoparone (SCOP), ethylmorphine (EtM), 1-naphthol (1-N) and phenol red (PhR) as test substrates, biotransformation activities were investigated in cultured hepatocytes from male and female rat, male and female goat, and female sheep and cattle. 2. As compared with rat hepatocytes, the total culture cytochrome P450 content was relatively well maintained in ruminant hepatocytes. In 72 h, it decreased to approximately half the initial content, whereas in rat hepatocytes only 30% was maintained. In ruminant hepatocytes, sulphation of 1-N remained fairly stable, glucuronidation of PhR decreased gradually, and glucuronidation of 1-N increased during the 72-h culture period. 3. Oxidative metabolism of TMP was rapid in goat and sheep hepatocytes, as compared with rat hepatocytes, reflecting species differences in TMP pharmacokinetics in vivo. In contrast with rat hepatocytes, 6-O-demethylation was by far the major pathway of scoparone metabolism in ruminant hepatocytes. The glucuronidation and sulphation activities were similar among the species. 4. In goat liver cells, sex differences in some oxidative biotransformations were observed, females being more active than males. In rat hepatocytes, a reverse sex difference was observed. 5. In conclusion, cultured hepatocytes from agricultural target species appear a useful in vitro model to study comparative metabolism of veterinary drugs and other xenobiotics. Comparing rat and ruminant, sex and species differences and similarities in drug metabolism can be observed that reflect the in vivo situation.

Cytotoxicity of menadione and related quinones in freshly isolated rat hepatocytes: effects on thiol homeostasis and energy charge

The cytotoxic events in freshly isolated rat hepatocytes following exposure over 2 h to menadione (2-methyl-1,4-naphthoquinone) and two closely related quinones, 2,3-dimethyl-1,4-naphthoquinone (DMNQ) and 1,4-naphthoquinone (NQ), were examined. These quinones differ in their arylation capacity (NQ > menadione >> DMNQ) and in their potential to induce redox cycling (NQ approximately menadione >> DMNQ) The glutathione status (reduced and oxidized glutathione) of the hepatocytes was determined using HPLC after derivatization with monobromobimane. Protein thiols were measured spectrophotometrically and the energy charge of the cells was determined with HPLC using ion pair chromatography. The leakage of lactate dehydrogenase was used as a marker for cell viability. All three quinones caused alterations of the glutathione status of the exposed cells but the effects were markedly different. Exposure to DMNQ resulted in a slow decrease of reduced glutathione and an increase of mixed disulfides. The other two quinones caused an almost complete depletion of reduced glutathione within 5 min. Hepatocytes exposed to NQ accumulated oxidized glutathione whereas menadione-exposed hepatocytes showed increased levels of mixed disulfides. We did not find any effects of DMNQ (200 microM) on protein thiols, energy charge or cell viability. There was a clear difference in the effects of menadione and NQ on protein thiols, energy charge and cell viability; exposure to NQ resulted in a more extensive decrease of protein thiols and energy charge and an earlier onset of lactate dehydrogenase leakage.(ABSTRACT TRUNCATED AT 250 WORDS)

Sulfadimidine metabolism in vitro: II. Comparative studies in cultured rat, goat, sheep and cattle hepatocytes

Hydroxylation and acetylation of sulphadimidine (SDD) and the deacetylation of N4-acetyl SDD was investigated in cultured hepatocytes from male and female rats, from male and female goats and from female sheep and cattle. Significant sex differences were observed for hydroxylation of SDD in hepatocytes from rat and goat. In goat, sheep and cow hepatocytes, the hydroxylation pathway is relatively important, whereas in rat hepatocytes, acetylation is predominant. Hepatocytes of all four species deacetylated N4-acetyl SDD. In ruminant hepatocytes, deacetylating activity was of considerable importance, whereas in rat hepatocytes, it appeared a minor pathway of metabolism. Similar to the in vivo situation, formation of N4-acetyl SDD in cultured hepatocytes results from an equilibrium of acetylation and deacetylation. A good correlation was found between results in isolated hepatocytes and previous findings in vivo, both in levels of species-related activities and in acetylation-hydroxylation ratios. In conclusion, cultured hepatocytes appear a useful in vitro model to study comparative sulfonamide metabolism.

Cytochrome P450 induction and metabolism of alkoxyresorufins, ethylmorphine and testosterone in cultured hepatocytes from goats, sheep and cattle

Very little is known of cytochrome P450 (P450) patterns and enzyme characteristics in food-producing animal species. Oxidative metabolism of alkoxyresorufins, ethylmorphine (EtM) and testosterone (TST) was used to monitor the effects of the P450 inducers phenobarbital (PB), beta-naphthoflavone (BNF), dexamethasone (DEX) and triacetyloleandomycin (TAO) in primary cultured hepatocytes from goats, sheep and cattle. BNF effectively and specifically induced ethoxyresorufin deethylase (> 20-fold), indicating the presence of an inducible P450 1A form, and down-regulated EtM demethylation and most selected TST hydroxylations. In non-induced hepatocyte cultures, TST was metabolized to 6 beta-, 2 beta-, 12 beta-, and 11 alpha-hydroxy-TST (OHT). PB and, to a lesser extent, DEX non-specifically induced all OHT formations, and EtM demethylation. TAO almost completely inhibited OHT formation and EtM demethylation. These results indicate the involvement of principally one P450 form, or a restricted number of related P450 forms, presumably belonging to the P450 3A subfamily. In western blot analysis, cross reactivity was found with rat anti-P450 3A1 and anti-sheep P450 3A. A more specific PB effect was observed for 16 alpha-OHT, which may be formed though a ruminant P450 2B form. None of the inducers influenced pentoxyresorufin depentylase (PROD) or EtM O-deethylation. Metabolite patterns and inducibility of selected activities in ruminant hepatocytes are in accordance with previous findings in goats in vivo. Cytochrome P450 characteristics in ruminants appear to differ from those in rats whereas similarities to the situation in humans appear to exist.

Sulphadimidine metabolism in vitro: I. Sex differences in acetylation and hydroxylation in cultured rat hepatocytes

The hydroxylation and acetylation of 0.5 mM sulphadimidine (SDD) was studied in primary cultures of hepatocytes from male and female rats, and from castrated male and sham operated male rats. In addition, SDD metabolism was investigated in hepatocytes from castrated male rats treated with testosterone, prior to liver cell isolation. In male rat hepatocytes a significantly higher hydroxylation activity was observed than in hepatocytes from female and castrated male rats. Acetylation activity was higher in females. Testosterone induced hydroxylation but did not affect acetylation. These results correlate well with data from previous in vivo studies, showing the relevance of this in vitro model.

Effects of the peroxisome proliferator mono(2-ethylhexyl)phthalate in primary hepatocyte cultures derived from rat, guinea pig, rabbit and monkey. Relationship between interspecies differences in biotransformation and peroxisome proliferating potencies

Primary hepatocyte cultures derived from rat, rabbit, guinea pig and monkey have been treated in vitro with metabolites of di(2-ethylhexyl)phthalate, i.e. mono(2-ethylhexyl)phthalate (MEHP), mono(5-carboxy-2-ethylpentyl)phthalate (metabolite V) and mono(2-ethyl-5-oxohexyl)phthalate (metabolite VI). In rat hepatocyte cultures MEHP and metabolite VI were equally potent in inducing peroxisome proliferation, while metabolite V was much less potent. In rat hepatocytes a 50% increase in both peroxisomal palmitoyl-CoA oxidase activity and microsomal lauric acid omega-hydroxylation activity was found after treatment with 5-15 microM MEHP. In guinea pig, rabbit and monkey hepatocyte cultures, a 50% increase in peroxisomal palmitoyl-CoA oxidase activity was found after treatment with 408-485 microM MEHP. No induction of lauric acid omega-hydroxylation activity was found. These results indicate that peroxisome proliferation can be induced by MEHP in rabbit, guinea pig and monkey hepatocytes, but that these species are at least 30-fold less sensitive to peroxisome proliferation induction than rats. The proposed mechanistic inter-relationship between induction of lauric acid omega-hydroxylation activity and peroxisome proliferation is found in rat hepatocytes, but not in hepatocytes of the other three species. Treatment of guinea pig hepatocyte cultures with MEHP resulted in an increase in triglyceride concentrations in the hepatocytes. In rat and rabbit hepatocyte cultures, triglyceride concentrations were much less altered by MEHP. In monkey hepatocytes a decrease in hepatic triglyceride concentration was found after treatment with MEHP. These effects are in agreement with in vivo effects observed before. After treatment of primary hepatocyte cultures with MEHP, high concentrations of omega- and (omega-1)-hydroxylated metabolites of MEHP were found in media from rat, rabbit and guinea pig cultures. The formation of these metabolites did not decline in time. During treatment the metabolite profile in media from rat hepatocyte cultures moved towards omega-hydroxy metabolites of MEHP. In media from monkey hepatocyte cultures the lowest concentrations of hydroxylated metabolites were determined. No major species differences were found in the potency to form oxidized MEHP metabolites, and thus no unique metabolite differences were found, which could explain the species differences in sensitivity for peroxisome proliferation.

Effect of diphenyl ether herbicides and oxadiazon on porphyrin biosynthesis in mouse liver, rat primary hepatocyte culture and HepG2 cells

The effects of the herbicides fomesafen, oxyfluorfen, oxadiazon and fluazifop-butyl on porphyrin accumulation in mouse liver, rat primary hepatocyte culture and HepG2 cells were investigated. Ten days of herbicide feeding (0.25% in the diet) increased the liver porphyrins in male C57B1/6J mice from 1.4 +/- 0.6 to 4.8 +/- 2.1 (fomesafen) 16.9 +2- 2.9 (oxyfluorfen) and 25.9 +/- 3.1 (oxadiazon) nmol/g wet weight, respectively. Fluazifop-butyl had no effect on liver porphyrin metabolism. Fomesafen, oxyfluorfen and oxadiazon increased the cellular porphyrin content of rat hepatocytes after 24 h of incubation (control, 3.2 pmol/mg protein, fomesafen, oxyfluorfen and oxadiazon at 0.125 mM concentration 51.5, 54.3 and 44.0 pmol/mg protein, respectively). The porphyrin content of HepG2 cells increased from 1.6 to 18.2, 10.6 and 9.2 pmol/mg protein after 24 h incubation with the three herbicides. Fluazifop-butyl increased hepatic cytochrome P450 levels and ethoxy- and pentoxyresorufin O-dealkylase (EROD and PROD) activity, oxyfluorfen increased PROD activity. Peroxisomal palmitoyl CoA oxidation increased after fomesafen and fluazifop treatment to about 500% of control values both in mouse liver and rat hepatocytes. Both rat hepatocytes and HepG2 cells can be used as a test system for the porphyrogenic potential of photobleaching herbicides.

Differences in the effects of model inducers of cytochrome P450 on the biotransformation of scoparone in rat and hamster liver

The hamster is known to display very high rates of monooxygenase-mediated biotransformation. In comparison with other species little knowledge has been gathered with respect to the nature of its cytochrome P450 enzymes and their respective inducibility. We studied the consequences of induction of P450 enzymes in rats and Syrian golden hamsters using the regioselective oxidative O-demethylation of the coumarin derivative scoparone. This metabolic conversion indicates differential effects of P450 inducers in the rat, in which various types of inducers cause different shifts in the isoscopoletin/scopoletin metabolite ratio (I/S-ratio). Liver microsomes from hamster not treated with P450 inducers oxidized scoparone much more efficiently than liver microsomes of untreated rats. In rat liver microsomes total demethylation rates of scoparone increased upon in vivo treatment with phenobarbital or beta-naphthoflavone. Phenobarbital reduced the I/S-ratio whereas beta-naphthoflavone caused an increase in this ratio. In hamster liver microsomes both phenobarbital and beta-naphthoflavone treatments resulted in a decrease in the I/S ratio. In this species the total scoparone demethylation rate was not much affected by phenobarbital, but beta-naphthoflavone caused a huge increase in over-all scoparone biotransformation. In both species, dexamethasone, isoniazid and clofibrate were much less effective. In contrast to the rat, in the hamster the scoparone biotransformation profile cannot be used to differentiate between phenobarbital- or beta-naphthoflavone-treated animals.

Hepatic cytochrome P450 induction in goats. Effects of model inducers on the metabolism of alkoxyresorufins, testosterone and ethylmorphine, and on apoprotein and mRNA levels

Male and female African dwarf goats were treated orally with phenobarbital (PB) or triacetyloleandomycin (TAO), or subcutaneously with beta-naphthoflavone (BNF). Hepatic microsomal cytochrome P450 content was increased by PB and TAO, but not by BNF. PB effects on P450 activities were non-selective: ethoxyresorufin deethylase (EROD) and pentoxyresorufin depentylase (PROD), hydroxylation of testosterone (TST) and demethylation of ethylmorphine (ETM) were all induced by a factor of 2-3. A similar non-selective induction was observed with TAO, except for EROD and PROD (no effects). After PB and TAO treatment, increased levels of a protein cross-reactive with anti-sheep P450 3A and 2B were found. Thus, in dwarf goats, both PB and TAO appeared to be P450 3A inducers. Selective PB effects related to a P450 2B form on PROD are lacking but 16 alpha-hydroxylation of TST was induced markedly. At the mRNA level, PB induced an mRNA that showed good sequence homology with a human P450 3A4 cDNA probe, rather than with a rat 3A1 probe. BNF selectively induced EROD, whereas TST hydroxylation and ETM dealkylation were inhibited. With BNF-treated animals, increased concentrations of a protein cross-reactive with anti-rat P450 1A1/1A2 and of an mRNA that showed homology with a human 1A1 cDNA probe, but not with a mouse 1A1/1A2 probe, were observed.

Effects of indole-3-carbinol on biotransformation enzymes in the rat: in vivo changes in liver and small intestinal mucosa in comparison with primary hepatocyte cultures

Groups of male Wistar rats were fed semi-synthetic diets containing 0, 200 or 500 mg indole-3-carbinol (13C)/kg for 2, 7, 14 or 28 days. After 2 days, P-450 activities were already induced, but the isoenzyme pattern induced was different in the liver and the small intestine. Hepatic P4501A1, P4501A2 and P4502B1 apoprotein levels were dose-relatedly enhanced, whereas in the small intestine induced levels of P4502B1 and P4501A1 were detected but P4501A2 was not induced. Pentoxy- and ethoxyresorufin dealkylation (PROD and EROD) were dose-relatedly enhanced in the liver (5- and 7-fold, respectively, in the higher dose group) as well as in the small intestine (8- and 13-fold, respectively, at 500 mg 13C/kg diet). Testosterone 16 alpha- and 16 beta-hydroxylation in the small intestine were enhanced (6-9-fold) from day 2 onwards, but in the liver these activities were only slightly enhanced from day 7 onwards. Thus, the major forms induced in the liver appear to be P4501A1, P4501A2, P4502B1 and, to a lesser extent, P4503A, whereas in the small intestine all of the effects that were found are associated with only one cytochrome P-450, P4502B1. After 2 days I3C (500 mg/kg) induced glutathione S-transferase in the liver (1.3-fold) and small intestine (1.5-fold). Hepatic glucuronyl transferase (GT1) was induced (about 1.6-fold) after 7, 14 and 28 days. DT-diaphorase was induced in the liver (2.7-fold) and small intestine (1.5-fold) after 14 days of exposure to 500 mg I3C/kg diet. Treatment of rat hepatocytes with indole-3-acetonitrile and 3,3'-diindolylmethane, but not I3C and indole-3-carboxaldehyde, enhanced EROD activity and halved testosterone 16 alpha- and 2 alpha-hydroxylation. All four indoles slightly induced glutathione S-transferase in cultured hepatocytes. Thus, the in vitro studies suggest that the in vivo effects of I3C have to be attributed to indole-condensation products, such as 3,3'-diindolylmethane, but not to I3C itself.

Effects of various medium formulations and attachment substrata on the performance of cultured ruminant hepatocytes in biotransformation studies

1. A procedure for the isolation and primary culture of hepatocytes from goat and cattle is described. Hepatocyte culture performance was monitored for 51 h by measuring viability, cytochrome P-450 maintenance, dealkylation of scoparone and ethylmorphine, and glucuronidation of phenol red. 2. Culture medium composition is discussed in relation to differences between splanchnic blood composition of ruminant and monogastric animal species. Main differences are in glucose and volatile fatty acid concentrations. Modified Williams' E culture medium did not yield higher culture performance than non-modified Williams' E. 3. Coating of culture dishes with either collagen or fibronectin did not improve culture performance. 4. Williams' E, although developed for rodent cells, proves to be a suitable basal medium for ruminant hepatocytes. In this medium, culture quality is high for at least several days. 5. In cultured goat hepatocytes, biotransformation rate for scoparone amounted to 20 nmol/mg protein per h, for ethylmorphine 96 nmol/mg protein per h and for phenol red 2 nmol/mg protein per h. Biotransformation activity in cow hepatocytes is approximately half that in goat hepatocytes.

Interlaboratory comparison of microsomal ethoxyresorufin and pentoxyresorufin O-dealkylation determinations: standardization of assay conditions

Assay conditions and results of cytochrome P-450 dependent 7-ethoxyresorufin (ER) and 7-pentoxyresorufin (PR) O-dealkylation (OD) by rat liver microsomes were compared by four laboratories in the Netherlands. Microsomal mixtures were prepared from control, 3-methylcholanthrene and phenobarbital pretreated animals, resulting in different levels of cytochrome P-450 isozymes. EROD and PROD activities were determined in each laboratory according to their own protocols. Considerable variability was found both between and within laboratories. Further studies demonstrated that protocol differences are important factors causing this interlaboratory variation. Main factors of influence were buffer type, batch of resorufin used for calibration, substrate solvent and substrate concentration. Based on the results obtained, standardized protocols for optimized measurement of microsomal EROD and PROD activities were developed. Additional experiments demonstrated that the use of these standardized protocols reduced intralaboratory variation in both the EROD and the PROD assay, whereas it also reduced the interlaboratory variability for the PROD determinations. The interlaboratory variation for measurement of microsomal EROD activities was only reduced for the laboratories using a Cobas-Bio analyzer. The results of the present study demonstrate clearly that data obtained with EROD and PROD activity measurements are highly sensitive to factors frequently varying from one laboratory to another. In addition, they demonstrate the necessity to be careful with absolute values presented in the literature for these activities, unless well characterized assay conditions are applied.

Acid reaction products of indole-3-carbinol and their effects on cytochrome P450 and phase II enzymes in rat and monkey hepatocytes

The effects of three acid condensation products of indole-3-carbinol (I3C), i.e. 3,3'-diindolylmethane (DIM), 5,6,11,12,17,18-hexahydrocyclonona[1,2-b:4,5-b':7,8-b"]tri-indole (CTI) and 2,3-bis[3-indolylmethyl]indole (BII), on cytochrome P450 and phase II enzymes were studied in primary cultures of rat and cynomolgus monkey liver cells. In rat hepatocytes all three indole derivatives dose-relatedly induced the ethoxyresorufin O-dealkylation (EROD) activity (to 24-fold) and 7 alpha-hydroxylation of testosterone (to 4-fold), whereas all three decreased the 16 alpha- and 2 alpha-testosterone hydroxylation (DIM to 60%, CTI and BII to a mere 5% of the control cells). Treatment of monkey hepatocytes with DIM and BII enhanced the EROD activity to 6- and 9-fold, respectively. Furthermore, BII decreased the 6 beta-hydroxylation of testosterone (to 60% of the untreated cultures) in monkey cells. Phase II enzymes were also affected. In rat hepatocytes DIM, CTI and BII enhanced DT-diaphorase (DTD) (= NAD(P)H-quinone reductase) activity, and DIM and BII the glucuronidation of 1-naphthol. In monkey cells BII only enhanced DTD, and no changes were observed in the glucuronidation of 1-naphthol after treatment with either DIM or BII. The indole derivatives did not affect glutathione S-transferase activity and sulfation of 1-naphthol in either rat or monkey hepatocytes. These results identify two novel acid condensation products of I3C, CTI and BII, as potent compounds in affecting biotransformation in rat as well as in monkey hepatocytes.

Induction of terminal differentiation in cultured human keratinocytes by polychlorinated aromatic hydrocarbons as measured by cell size analysis

Polychlorinated aromatic hydrocarbons modulate the proliferation and differentiation of human epidermal cells in vivo and in culture. One of the earliest events in the process of terminal differentiation is the increase in cell size. In this report the usefulness of morphometric cell size analysis as a quantifiable marker for chemical-induced differentiation was examined. Concentration-related increases in cell size distribution were induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) and 2,3,4,7,8-pentachlorodibenzofuran in normal human keratinocytes and cells from an SV40-transformed keratinocyte cell line (SVK14) whereas the analog 1,2,3,4-tetrachlorodibenzo-p-dioxin did not affect the cell size distribution up to a concentration of 100 nM. The minimal effective concentrations of five 2,3,7,8-substituted polychlorinated dibenzo-p-dioxins/dibenzofurans and a coplanar polychlorbiphenyl necessary to induce an increase in cell size distribution were determined in SVK14 cells. It was found that the potency of these compounds relative to that of 2,3,7,8-TCDD correlated well with the toxicity equivalency factors observed in other test systems. This indicates that the keratinocyte cell assay is a useful method for establishing the relative potency of various "dioxins" and their mixtures.

Effects of cooked brussels sprouts on cytochrome P-450 profile and phase II enzymes in liver and small intestinal mucosa of the rat

Male Wistar rats were given semi-synthetic diets supplemented with 0, 2.5, 5 and 20% cooked Brussels sprouts for 2, 7, 14 or 28 days. The effects on several cytochrome P-450 enzymes and phase II enzymes (glutathione S-transferase (GST), glucuronyl transferases 1 and 2 (GT1 and GT2) and DT-diaphorase (DTD)) in the liver and small intestinal mucosa were investigated. From 2 days of exposure onwards Brussels sprouts induced P4501A2 and--to a lesser extent--P4501A1 apoprotein levels in the liver, whereas in the small intestine markedly enhanced P4502B apoprotein levels could be detected. No enhanced P4503A apoprotein levels were observed. The 5 and 20% sprouts diets increased the intestinal pentoxyresorufin depentylation (PROD, 4.5-9-fold), and the hydroxylation of testosterone at the 16 alpha- and 16 beta-site (2.6-4.2-fold) after 2 days of exposure. In addition, the 20% sprouts died also enhanced the intestinal ethoxyresorufin deethylation (EROD) activity (c. 5-fold), the hepatic EROD and PROD activities (c. 2-fold) and the formation of 6 beta-hydroxytestosterone (c. 1.6-fold); the formation of 2 alpha-hydroxytestosterone in the liver was decreased (to c. 70% of the control value). GST activity was induced both in the liver (5 and 20% diet) and intestine (20% diet only) throughout the experiment. The 20% sprouts diet enhanced the hepatic DTD and GT1 activities, whereas the GT2 activity was decreased. The induction of DTD in the small intestine after 2 days (2.5-3.2-fold with 5 and 20% sprouts diets, respectively) diminished during the experiment. These results indicate that dietary exposure to cooked Brussels sprouts for only 2 days can change the metabolic activities of several phase II enzymes and cytochrome P-450 enzymes, of which P4502B is the predominant form induced in the small intestine.

Comparison of cytochrome P450 isoenzyme profiles in rat liver and hepatocyte cultures. The effects of model inducers on apoproteins and biotransformation activities

The metabolic profile of seven subfamilies of cytochrome P450 (P450IA, IIA, IIB, IIC, IIE, IIIA, IVA) was studied in rat liver (in vivo) and in primary hepatocyte cultures (in vitro) after treatment with various inducers. The dealkylation of 7-ethoxyresorufin (EROD) and 7-pentoxyresorufin (PROD), aniline 4-hydroxylation and the regio- and stereoselective hydroxylation of testosterone were measured to characterize the isoenzyme pattern in intact hepatocytes and in liver microsomes. Occurrence of isoenzyme apoproteins was determined using Western blotting. Primary cultures of rat hepatocytes retain the capacity to respond to inducers of isoenzymes belonging to six different subfamilies (P450IA, IIA, IIB, IIC, IIIA and IVA). Treatment of cells with beta-naphthoflavone revealed a P450-activity profile similar to in vivo, namely a highly induced EROD (P450IA1), a small enhancement of testosterone 7 alpha-hydroxylation (P450IIA) and a marked reduction in 2 alpha- and 16 alpha-hydroxylation (P450IIC11). Exposure of cultured cells to phenobarbital resulted in a higher testosterone 16 beta-hydroxylation (reflecting P450IIB), though to a lesser extent than in vivo. The induction of P450IIIA due to both phenobarbital and dexamethasone, as mirrored by 6 beta- and 15 beta-hydroxylation of testosterone, was the same in cultured hepatocytes and in vivo. Treatment of cells with clofibric acid resulted in an induction profile similar to the one observed in liver microsomes from clofibrate-treated rats: the apoprotein P450IVA as well as the apoprotein P450IIB1/2 and its associated activities (PROD and testosterone 16 beta-hydroxylation) were induced. Isoniazid, a known in vivo inducer of P450IIE1 and aniline 4-hydroxylation, did not change any of the determined P450-dependent activities in vitro.

Biotransformation of scoparone used to monitor changes in cytochrome P450 activities in primary hepatocyte cultures derived from rats, hamsters and monkeys

The coumarin derivative scoparone is regioselectively demethylated yielding isoscopoletin and scopoletin. The ratio of the formation rates of these two metabolites (isoscopoletin/scopoletin; I/S ratio) is reported to mirror the contribution of several cytochrome P450 (P450) isoenzymes to the biotransformation of scoparine. The metabolism of scoparine has been studied in primary liver cell cultures of rats, hamsters, cynomolgus monkeys and in human liver cells. Rat hepatocytes appeared to metabolize scoparone 7 to 10 times slower than those of hamsters and monkeys. In hepatocyte monolayers of all three species the loss of P450 was paralleled by a decrease in total scoparone metabolism. In hamsters but not in rats, a decrease of the I/S ratio was found during primary culture of liver cells. A similar shift in the metabolic pattern of scoparine observed with the monkey hepatocytes was statistically not significant. Most likely, in hamster and possibly in monkey hepatocyte cultures the different P450s involved in scoparone metabolism decrease at unequal rates. In rat liver cells, however, the pattern of these P450 isoenzymes remains more or less unaltered. In contrast to liver cells from the other species, human hepatocytes did not secrete scopoletin in detectable amounts. Scoparone demethylation in humans may be qualitatively different from that in other mammals.

Structure elucidation of acid reaction products of indole-3-carbinol: detection in vivo and enzyme induction in vitro

The potency of indole-3-carbinol (I3C) to form condensation products under acidic aqueous conditions was studied. After identifying a known dimer, 3,3'-diindolylmethane (DIM), we elucidated the structures of two trimers also found in acid reaction mixtures: 5,6,11,12,17,18-hexahydrocyclonona[1,2-b:4,5-b':7,8-b"]tri-indole (CTI), and 2,3-bis[3-indolylmethyl] indole (BII). The formation of these indole oligomers was shown to be pH dependent. The highest amounts of DIM and BII were formed in aqueous solutions having a pH value ranging from 4 to 5. No CTI could be detected at pH values above 4.5. In rats that received an oral dose of I3C we could detect DIM and BII in gastric contents, stomach tissue, small intestine and liver. No CTI could be detected in vivo after oral exposure to I3C. In in vitro experiments, using rat hepatocytes, the cytochrome P-450IA1 apoprotein level, 7-ethoxyresorufin O-deethylation activity (EROD) and DT-diaphorase activity (DTD) were markedly enhanced by DIM and CTI as well as BII.

Immunohistochemical detection of cytochrome P450 isoenzymes in cultured human epidermal cells

We used specific monoclonal antibodies (MAb) to human cytochrome P450 isoenzymes to determine the presence of these proteins in human epidermal cells. Two MAb (P450-5 and P450-8) recognize major forms of hepatic cytochrome P450 involved in biotransformation of xenobiotics. A third MAb, to cytochrome P450-9, is not fully characterized. The proteins were determined by the indirect immunoperoxidase technique after fixation with methanol and acetone. Biopsy materials for cultured keratinocytes, i.e., foreskin and hair follicles, contained the two major forms of cytochrome P450. In cultured keratinocytes derived from hair follicles the proteins were undetectable, whereas the keratinocytes derived from foreskin continued to express the two major forms of hepatic cytochrome P450. Cultured human fibroblasts and a human keratinocyte cell line (SVK14) showed staining similar to that of the foreskin keratinocytes. Cytochrome P450-9 was detectable only in human hepatocytes. The results indicate that, under the culture conditions applied, cultured human foreskin cells and the cell line SVK14 continue to express specific cytochrome P450 isoenzymes in culture, in contrast to hair follicle keratinocytes.

The isoenzyme pattern of cytochrome P450 in rat hepatocytes in primary culture, comparing different enzyme activities in microsomal incubations and in intact monolayers

Changes in the isoenzyme pattern of cytochrome P450 during culture were investigated in primary cultures of rat hepatocytes, measuring specific enzyme activities in microsomes prepared from cultured cells as well as in intact monolayers. Assays of 7-ethoxyresorufin O-deethylation (EROD), 7-pentoxyresorufin O-depentylation (PROD), aniline 4-hydroxylation (AH) and the specific regioselective hydroxylation of testosterone were used as representatives of the activities of seven isoenzymes of cytochrome P450. The isoenzyme profile expressed as catalytic activities was qualitatively and quantitatively similar in microsomes obtained from freshly isolated hepatocytes in comparison with microsomes obtained from whole livers of untreated rats. There was a relatively high activity in EROD, AH and the oxidation of testosterone at the 7 alpha, 2 alpha, 6 beta, 16 alpha and 17 sites (androstenedione). During culture, these microsomal enzyme activities declined at a similar rate to ca. 50% of the activities of microsomes prepared from freshly isolated hepatocytes after 24 hr and to 15% after 96 hr. The overall decline of cytochrome P450-dependent activities during culture was not accompanied with gross changes in catalytic profile. Determining the same drug-metabolizing activities directly in intact hepatocyte monolayers revealed a much higher metabolic rate for all measured P450-dependent activities. The profile of the catalytic activities was essentially the same as measured in microsomes prepared from cultured hepatocytes. The relatively low activity towards the 7 alpha site of testosterone measured in intact hepatocytes, however, remained constant during culture. Determination of enzyme activities directly in intact hepatocytes is a convenient way of studying changes in monooxygenase activities of different P450 isoenzymes in vitro.

The effect of beclobric acid and clofibric acid on peroxisomal beta-oxidation and peroxisome proliferation in primary cultures of rat, monkey and human hepatocytes

The peroxisome-proliferating effects of clofibric acid and beclobric acid were studied in primary cultures of hepatocytes derived from rat, monkey (Macaca fascicularis) and human liver. Determination of peroxisomal fatty acid beta-oxidation and morphometrical analysis of the peroxisomal compartment were performed after incubation of 1-day-old hepatocyte cultures for 3 days with either compound. In rat liver cell cultures both compounds gave a 10-fold increase in peroxisomal beta-oxidation, a 3-fold increase in the relative number of peroxisomes and a 1.5-fold increase in the mean size of peroxisomes. Beclobric acid gave its maximal effect at a concentration of 10 microM, which is at least one order of magnitude lower than the maximum-effect concentration of clofibric acid. At concentrations greater than 300 microM beclobric acid was cytotoxic. No stimulation of peroxisomal fatty acid beta-oxidation was found in either monkey or human hepatocyte cultures. Morphometrical analysis also showed no increase in the peroxisomal compartment in cultures derived from these species, as indicated by the lack of increase in both relative number and size of peroxisomes. In all three species tested beclobric acid was equally cytotoxic for hepatocytes in vitro. These results are of relevance for the interpretation of the peroxisome-proliferating effects of clofibrate and similar compounds in rats. Since peroxisome proliferation may be correlated to increased hepatic tumour incidences in the rat, the absence of peroxisome proliferation in primates suggests the absence of tumourogenic activity by hypolipidemic compounds in these species.

Effects of sulphydryl reagents on the formation of the aniline metabolite 4-aminophenol and its sulphate and glucuronide conjugates in primary cultures of rat hepatocytes

1. The effects of various sulphydryl-blocking reagents on aniline biotransformation and cytochrome P-450 levels were studied in cultured rat hepatocytes. 2. Exposure of aniline-metabolizing hepatocytes to p-chloro-mercuribenzoate (PCMB) or p-chloromercuribenzenesulphonic acid (PCMBS) resulted in decreased levels of cytochrome P-450, decreased glucuronidation of 4-aminophenol and increased levels of free 4-aminophenol. 3. Incubation of aniline-metabolizing hepatocytes with disulfiram resulted in decreased formation of 4-aminophenol, but this was not associated with impaired glucuronidation or cytochrome P-450 levels. 4. Exposure of aniline-metabolizing hepatocytes to mersalyl, 2,2'-dithiodipyridine (DTP), 6,6'-carboxydipyridine disulphide (CPDS) or N-ethylmaleimide did not affect the biotransformation of aniline or cytochrome P-450 levels. 5. Metyrapone prevented degradation of cytochrome P-450. Exposure of cells to SKF-525 A inhibited aniline biotransformation without altering cytochrome P-450 levels. 6. PCMB and PCMBS inhibited aniline metabolism, probably by binding to a cysteinyl-SH residue in cytochrome P-450 apoenzyme and 'active sites' of UDP-glucuronyl transferases. Disulfiram inhibited aniline biotransformation, probably indirectly by diminishing NADPH.

Rapid method for the determination and quantification of bromosulphophthalein and metabolites in cultured hepatocytes, culture media and bile by high-performance liquid chromatography

An ion-pair high-performance liquid chromatographic method for the rapid, selective and sensitive analysis of samples containing bromosulphophthalein (BSP) and its conjugates is presented. The method is useful for analysis in bile, culture media and cultured hepatocytes. Two sample preparation methods are described. Even though BSP recovery from albumin binding is complete, only a small percentage of free BSP can be detected in cells, possibly owing to a conjugation-related pool of BSP in cells. As BSP-glutathione recovery is complete, the method offers a useful tool to investigate impairment of glutathione conjugation.

Effects of 1,2-dibromoethane on isolated hepatocytes: functional alterations and induction of lipid peroxidation

1. Isolated rat hepatocytes were exposed to the fumigant 1,2-dibromoethane (DBE) and cytotoxicity was evaluated by studying parameters of cellular function and lipid peroxidation. 2. DBE caused plasma membrane damage, as determined by leakage of lactate dehydrogenase, and was more severe in shaken suspensions than stationary suspensions, suggesting that cells were more fragile after DBE exposure. 3. DBE decreased hepatocyte glycogen content and stimulated albumin synthesis in hepatocyte suspensions. 4. Lipid peroxidation resulted from DBE exposure and was greater in cells isolated from phenobarbital-pretreated rats. Shaking the suspensions enhanced lipid peroxidation. Ethane production did not parallel formation of thiobarbituric acid reactants, suggesting that these parameters of lipid peroxidation reflect different mechanisms of molecular interaction of DBE.

Interlaboratory comparison of total cytochrome P-450 and protein determinations in rat liver microsomes. Reinvestigation of assay conditions

Assay conditions in determining total cytochrome P-450 in four laboratories were compared. Although the determination was derived from the original Omura and Sato method in each laboratory, the four standard protocols differed slightly, resulting in considerable differences in the results. Since the cytochrome P-450 content is usually expressed per mg protein, the protein assay conditions were evaluated as well. Furthermore, we compared the cytochrome P-450 values obtained by the CO- and the dithionite (DT)-difference methods. The effect of a number of variables in the assay was investigated. The influence of the storage temperature of the microsomes was ascertained as well as effects of the gassing time with CO and the time between addition of dithionite, CO-gassing and the recording of the difference spectra. After evaluating these variables a standard operation procedure was established. Using this procedure the interlaboratory coefficient of variation for total cytochrome P-450 was 4.8%, a value which was comparable to the intralaboratory coefficients of variation. The final results also show that the millimolar extinction coefficient for the DT-difference method is higher than for the CO-difference method.

NADH-ferrihemoglobin reductase in avian erythrocytes

The assay for NADH-ferrihemoglobin reductase (NADH-FR) was optimized for avian blood samples. In this assay the pH optimum for Japanese quail red cell NADH-FR was 5.5, which was close to the enzyme's pI. Enzyme kinetic parameters were determined for quail, chicken and turkey NADH-FR. Preparation of erythrocyte ghost-cells and subsequent fractionation showed that the enzyme was present in the plasma membrane as well as in the nuclear membrane, while Triton X-100 treatment gave a release of enzyme activity from the membrane. In the cytosolar fraction of avian red cells no NADH-FR could be detected.

Kinetics of the formation and secretion of the aniline metabolite 4-aminophenol and its conjugates by isolated rat hepatocytes

The rates of secretion of 4-aminophenol and its sulphate and glucuronide conjugate were determined in cultures of rat hepatocytes with aniline and 4-aminophenol as substrates. When 4-aminophenol (300 microM) was used as substrate, 4-aminophenol disappeared from the medium within 30 min. Secretion of conjugates continued for more than 60 min, when 70% of the 4-aminophenol had been secreted as conjugates. At a low concn. of 4-aminophenol, the sulphate ester was the main metabolite, while secretion of the glucuronide showed a more than proportional rise ('lag phase') with increasing substrate concn. At higher concn. (greater than 300 microM) about equal amounts of both conjugates were formed. Without inorganic sulphate, sulphation of 4-aminophenol was greatly diminished and the lag phase in glucuronide secretion was not found. With 1 mM aniline as substrate up to 300 microM of conjugated 4-aminophenol was secreted with a linear time-dependence for at least two hours. With aniline as substrate the sulphate ester was the most important conjugate and lag phases in the secretion of both conjugates were minimal. Phenobarbitone pretreatment in vivo stimulated the secretion of conjugated products after incubation with aniline. No dramatic changes in the profile of the lag phases were seen. The differences in the conjugation profiles of both substrates can be explained by taking into consideration the differences in the expected intracellular concentrations.

Formation and disposition of N-hydroxylated metabolites of aniline and nitrobenzene by isolated rat hepatocytes

The formation and secretion of phenylhydroxylamine plus nitrosobenzene was studied in cultures of isolated rat hepatocytes after addition of aniline or nitrobenzene. With aniline concn. up to 10 mM, N-oxygenated metabolites were secreted linearly with time over 2 h. Phenobarbitone pretreatment in vivo for c. 5 d increased aniline N-hydroxylation by a factor of 2.8. Nitrobenzene reduction by isolated rat hepatocytes, yielding phenylhydroxylamine plus nitrosobenzene in the medium, was stimulated 1.9-fold and 4.3-fold after phenobarbitone pretreatment in vivo for 5 and 10 d, respectively. After reduction of nitrobenzene by isolated hepatocytes, the secretion of N-oxygenated products into the medium was non-linear with time for substrate concn. higher than 2.5 mM, probably due to the formation of cytotoxic concn. of nitrosobenzene. Isolated rat hepatocytes reduced phenylhydroxylamine to aniline. Results indicate that isolated rat hepatocytes are a reliable and sensitive system to demonstrate N-oxygenated metabolites of aromatic amino- and reduction of nitrocompounds.

Reversed phase high-performance liquid chromatography of heterocyclic aromatic N-oxides: application to measurement of N-oxidation by microsomes and rat hepatocytes in primary culture

1. The rates of N-oxide formation from pyridine, nicotinamide and isonicotinamide by rat liver microsomes and by cultured rat hepatocytes have been quantified using a simple h.p.l.c. system. 2. Hepatocytes produce hetero-aromatic N-oxides for at least 5 h, making them a useful system to study the N-oxidation of pyridines.

Effects of phenylhydroxylamine and aminophenols in Japanese quail in vivo

1. The effects of phenylhydroxylamine, and o- and p-aminophenol were studied in the Japanese quail. 2. Symptoms normally observed in aniline-treated birds were seen in quail after phenylhydroxylamine dosage at > 10 mg/kg. Aminophenols (up to 50 mg/kg) did not give these symptoms. 3. Injection of phenylhydroxylamine (50 mg/kg) resulted in formation of 70% ferrihaemoglobin after 5 min, following which a rapid reduction of ferrihaemoglobin was observed. 4. Phenylhydroxylamine reached highest blood concn. of 0.2 mumol/ml after 5 min. Phenylhydroxylamine was reduced to aniline within 5 min. 5. The effects of aniline in vivo are most probably due to O2 shortage caused by ferrihaemoglobin formation.

Biochemical processes involved in ferrihemoglobin formation by monohydroxyaniline derivatives in erythrocytes of birds and mammals

Background

Major depressive disorder (MDD) is the leading cause of global disability; however, the existing treatments do not always address cognitive dysfunction—a core feature of MDD. Immersive virtual reality (VR) has emerged as a promising modality to enhance the real-world effectiveness of cognitive remediation.

Objective

This study aimed to develop the first prototype VR cognitive remediation program for MDD (“bWell-D”). This study gathered qualitative data from end users early in the design process to enhance its efficacy and feasibility in clinical settings.

Methods

Semistructured end-user interviews were conducted remotely (n=15 patients and n=12 clinicians), assessing the participants’ perceptions and goals for a VR cognitive remediation program. Video samples of bWell-D were also shared to obtain feedback regarding the program. The interviews were transcribed, coded, and analyzed via thematic analysis.

Results

End users showed an optimistic outlook toward VR as a treatment modality, and perceived it as a novel approach with the potential of having multiple applications. The participants expressed a need for an engaging VR treatment that included realistic and multisensorial settings and activities, as well as customizable features. Some skepticism regarding its effectiveness was also reported, especially when the real-world applications of the practiced skills were not made explicit, as well as some concerns regarding equipment accessibility. A home-based or hybrid (ie, home and clinic) treatment modality was preferred.

Conclusions

Patients and clinicians considered bWell-D interesting, acceptable, and potentially feasible, and provided suggestions to enhance its real-world applicability. The inclusion of end-user feedback is encouraged when developing future VR programs for clinical purposes.