Validation of whole chick embryo cultures, whole rat embryo cultures and aggregating embryonic brain cell cultures using six pairs of coded compounds

A comparative study was performed to assess the effects of six pairs of coded compounds using cultures of whole chick and rat embryos as well as aggregating brain cell cultures. Developed originally for basic studies in developmental biology, these three culture systems have been adapted for the screening of chemicals in the field of prenatal toxicology. Chick and rat embryos were cultured for 2 days during the early stages of organogenesis. Aggregating cell cultures were prepared from early foetal rat telecephalon and grown for 14 days in a chemically defined medium. Concentration-response relationships were established by treating whole embryos in vitro for 2 days, and aggregating brain cell cultures for 9 days. After decoding the compounds, the results showed that, in the three test systems, specific effects were induced at comparable concentration levels. Similar compound-related malformations could be observed in both chick and rat whole embryo cultures. In aggregating brain cell cultures, neuron- and glia-specific effects could be distinguished. Based on the results obtained in the three in vitro systems, the following concentration ranges were determined for the teratogenic/toxic potencies of the test compounds (in mol/litre): <10(-6): retinoids (Ro 13-6307, Ro 1-5488), 6-aminonicotinamide, ketoconazole; 10(-6)-10(-3): 4-hydroxypyridine, sulfadiazine, sulfanilamide, caffeine, theophylline, metronidazole, methoxyacetic acid; >10(-3): methoxyethanol. In general, the three in vitro test systems were found to provide concordant and complementary data on the toxicity and teratogenicity of a given compound. These data were also comparable with those available from in vivo studies. It is therefore concluded that such a test battery could contribute significantly to risk assessment and to the reduction of in vivo experimentation in reproductive toxicology.

Evidence for a role of oxidative stress in the carcinogenicity of ochratoxin a

The in vitro and in vivo evidence compatible with a role for oxidative stress in OTA carcinogenicity has been collected and described. Several potential oxido-reduction mechanisms have been identified in the past. More recently, the possibility of a reduction of cellular antioxidant defense has been raised as an indirect source of oxidative stress. Consequences resulting from the production of oxidative stress are observed at different levels. First, OTA exposure has been associated with increased levels of oxidative DNA, lipid, and protein damage. Second, various biological processes known to be mobilized under oxidative stress were shown to be altered by OTA. These effects have been observed in both in vitro and in vivo test systems. In vivo, active doses were often within doses documented to induce renal tumors in rats. In conclusion, the evidence for the induction of an oxidative stress response resulting from OTA exposure can be considered strong. Because the contribution of the oxidative stress response in the development of cancers is well established, a role in OTA carcinogenicity is plausible. Altogether, the data reviewed above support the application of a threshold-based approach to establish safe level of dietary human exposure to OTA.

Predictive models for carcinogenicity and mutagenicity: frameworks, state-of-the-art, and perspectives

Mutagenicity and carcinogenicity are endpoints of major environmental and regulatory concern. These endpoints are also important targets for development of alternative methods for screening and prediction due to the large number of chemicals of potential concern and the tremendous cost (in time, money, animals) of rodent carcinogenicity bioassays. Both mutagenicity and carcinogenicity involve complex, cellular processes that are only partially understood. Advances in technologies and generation of new data will permit a much deeper understanding. In silico methods for predicting mutagenicity and rodent carcinogenicity based on chemical structural features, along with current mutagenicity and carcinogenicity data sets, have performed well for local prediction (i.e., within specific chemical classes), but are less successful for global prediction (i.e., for a broad range of chemicals). The predictivity of in silico methods can be improved by improving the quality of the data base and endpoints used for modelling. In particular, in vitro assays for clastogenicity need to be improved to reduce false positives (relative to rodent carcinogenicity) and to detect compounds that do not interact directly with DNA or have epigenetic activities. New assays emerging to complement or replace some of the standard assays include Vitotox, GreenScreenGC, and RadarScreen. The needs of industry and regulators to assess thousands of compounds necessitate the development of high-throughput assays combined with innovative data-mining and in silico methods. Various initiatives in this regard have begun, including CAESAR, OSIRIS, CHEMOMENTUM, CHEMPREDICT, OpenTox, EPAA, and ToxCast. In silico methods can be used for priority setting, mechanistic studies, and to estimate potency. Ultimately, such efforts should lead to improvements in application of in silico methods for predicting carcinogenicity to assist industry and regulators and to enhance protection of public health.

MAPK-ERK activation in kidney of male rats chronically fed ochratoxin A at a dose causing a significant incidence of renal carcinoma

Kidney samples of male Fischer 344 (F-344) rats fed a carcinogenic dose of OTA over 7 days, 21 days and 12 months were analysed for various cell signalling proteins known to be potentially involved in chemical carcinogenicity. OTA was found to increase the phosphorylation of atypical-PKC. This was correlated with a selective downstream activation of the MAP-kinase extracellular regulated kinases isoforms 1 and 2 (ERK1/2) and of their substrates ELK1/2 and p90RSK. Moreover, analysis of effectors acting upstream of PKC indicated a possible mobilisation of the insulin-like growth factor-1 receptor (lGFr) and phosphoinositide-dependent kinase-1 (PDK1) system. An increased histone deacetylase (HDAC) enzymatic activity associated with enhanced HDAC3 protein expression was also observed. These findings are potentially relevant with respect to the understanding of OTA nephrocarcinogenicity. HDAC-induced gene silencing has previously been shown to play a role in tumour development. Furthermore, PKC and the MEK-ERK MAP-kinase pathways are known to play important roles in cell proliferation, cell survival, anti-apoptotic activity and renal cancer development.

A Toxicogenomics Approach to Identify New Plausible Epigenetic Mechanisms of Ochratoxin A Carcinogenicity in Rat

Ochratoxin A (OTA) is a mycotoxin occurring naturally in a wide range of food commodities. In animals, it has been shown to cause a variety of adverse effects, nephrocarcinogenicity being the most prominent. Because of its high toxic potency and the continuous exposure of the human population, OTA has raised public health concerns. There is significant debate on how to use the rat carcinogenicity data to assess the potential risk to humans. In this context, the question of the mechanism of action of OTA appears of key importance and was studied through the application of a toxicogenomics approach. Male Fischer rats were fed OTA for up to 2 years. Renal tumors were discovered during the last 6 months of the study. The total tumor incidence reached 25% at the end of the study. Gene expression profile was analyzed in groups of animals taken in intervals from 7 days to 12 months. Tissue-specific responses were observed in kidney versus liver. For selected genes, microarray data were confirmed at both mRNA and protein levels. In kidney, several genes known as markers of kidney injury and cell regeneration were significantly modulated by OTA. The expression of genes known to be involved in DNA synthesis and repair, or genes induced as a result of DNA damage, was only marginally modulated. Very little or no effect was found amongst genes associated with apoptosis. Alterations of gene expression indicating effects on calcium homeostasis and a disruption of pathways regulated by the transcription factors hepatocyte nuclear factor 4 alpha (HNF4alpha) and nuclear factor-erythroid 2-related factor 2 (Nrf2) were observed in the kidney but not in the liver. Previous data have suggested that a reduction in HNF4alpha may be associated with nephrocarcinogenicity. Many Nrf2-regulated genes are involved in chemical detoxication and antioxidant defense. The depletion of these genes is likely to impair the defense potential of the cells, resulting in chronic elevation of oxidative stress in the kidney. The inhibition of defense mechanism appears as a highly plausible new mechanism, which could contribute to OTA carcinogenicity.

Inhibition of the expression and activity of cyclooxygenase-2 by chicory extract

Chicory is a major source of fructans with reported prebiotic-bifidogenic properties. In the present study, the potential anti-inflammatory activities of chicory were investigated. Ethyl acetate chicory root extract produced a marked inhibition of prostaglandin E(2) (PGE(2)) production in human colon carcinoma HT29 cells treated with the pro-inflammatory agent TNF-alpha. Two independent mechanisms of action were identified: (1) a drastic inhibition of the induction by TNF-alpha of cyclooxygenase 2 (COX-2) protein expression and (2) a direct inhibition of COX enzyme activities with a significantly higher selectivity for COX-2 activity. The inhibition of TNF-alpha-dependent induction of COX-2 expression was mediated by an inhibition of NF-kappaB activation. A major sesquiterpene lactone of chicory root, the guaianolide 8-deoxylactucin, was identified as the key inhibitor of COX-2 protein expression present in chicory extract. Altogether, the data presented strongly support chicory root as a promising source of functional food ingredient, combining prebiotic and anti-inflammatory properties.

Involvement of glial cells in the neurotoxicity of parathion and chlorpyrifos

An in vitro model, the aggregating brain cell culture of fetal rat telencephalon, has been used to investigate the influence of glial cells on the neurotoxicity of two organophosphorus pesticides (OPs), chlorpyrifos and parathion. Mixed-cell aggregate cultures were treated continuously for 10 days between DIV 5 and 15. Parathion induced astrogliosis at concentration at which MAP-2 immunostaining, found here to be more sensitive than neuron-specific enzyme activities, was not affected. In contrast, chlorpyrifos induced a comparatively weak gliotic reaction, and only at concentrations at which neurons were already affected. After similar treatments, increased neurotoxicity of parathion and chlorpyrifos was found in aggregate cultures deprived of glial cells. These results suggest that glial cells provide neuroprotection against OPs toxicity. To address the question of the difference in toxicity between parathion and chlorpyrifos, the toxic effects of their leaving groups, p-nitrophenol and trichloropyridinol, were studied in mixed-cell aggregates. General cytotoxicity was more pronounced for trichloropyridinol and both compounds had similar toxic effects on neuron-specific enzyme activities. In contrast, trichloropyridinol induced a much stronger decrease in glutamine synthetase activity, the enzymatic marker of astrocytes. Trichloropyridinol may exert a toxic effect on astrocytes, compromising their neuroprotective function, thus exacerbating the neurotoxicity of chlorpyrifos. This is in line with the suggestion that glial cells may contribute to OPs neurotoxicity, and with the view that OPs may exert their neurotoxic effects through different mechanisms.

Structure-based thresholds of toxicological concern (TTC): guidance for application to substances present at low levels in the diet

The threshold of toxicological concern (TTC) is a pragmatic risk assessment tool that is based on the principle of establishing a human exposure threshold value for all chemicals, below which there is a very low probability of an appreciable risk to human health. The concept that there are levels of exposure that do not cause adverse effects is inherent in setting acceptable daily intakes (ADIs) for chemicals with known toxicological profiles. The TTC principle extends this concept by proposing that a de minimis value can be identified for many chemicals, in the absence of a full toxicity database, based on their chemical structures and the known toxicity of chemicals which share similar structural characteristics. The establishment and application of widely accepted TTC values would benefit consumers, industry and regulators. By avoiding unnecessary toxicity testing and safety evaluations when human intakes are below such a threshold, application of the TTC approach would focus limited resources of time, cost, animal use and expertise on the testing and evaluation of substances with the greatest potential to pose risks to human health and thereby contribute to a reduction in the use of animals. An Expert Group of the European branch of the International Life Sciences Institute-ILSI Europe-has examined the TTC principle for its wider applicability in food safety evaluation. The Expert Group examined metabolism and accumulation, structural alerts, endocrine disrupting chemicals and specific endpoints, such as neurotoxicity, teratogenicity, developmental toxicity, allergenicity and immunotoxicity, and determined whether such properties or endpoints had to be taken into consideration specifically in a step-wise approach. The Expert Group concluded that the TTC principle can be applied for low concentrations in food of chemicals that lack toxicity data, provided that there is a sound intake estimate. The use of a decision tree to apply the TTC principle is proposed, and this paper describes the step-wise process in detail. Proteins, heavy metals and polyhalogenated-dibenzodioxins and related compounds were excluded from this approach. When assessing a chemical, a review of prior knowledge and context of use should always precede the use of the TTC decision tree. The initial step is the identification and evaluation of possible genotoxic and/or high potency carcinogens. Following this step, non-genotoxic substances are evaluated in a sequence of steps related to the concerns that would be associated with increasing intakes. For organophosphates a TTC of 18microg per person per day (0.3 microg/kg bw/day) is proposed, and when the compound is not an OP, the TTC values for the Cramer structural classes III, II and I, with their respective TTC levels (e.g. 1800, 540 and 90 microg per person per day; or 30, 9 and 1.5 microg/kg bw /day), would be applied sequentially. All other endpoints or properties were shown to have a distribution of no observed effect levels (NOELs) similar to the distribution of NOELs for general toxicity endpoints in Cramer classes I, II and III. The document was discussed with a wider audience during a workshop held in March 2003 (see list of workshop participants).

Guidance for the safety assessment of botanicals and botanical preparations for use in food and food supplements

There is a growing interest by both consumers and industry for the development of food products with 'functional' properties, or health benefits. These products may take the form of dietary supplements or of foods. The health benefits are given by particular ingredients, and in many cases these are derived from botanicals. The variety of plants providing these functions is large, ranging from staple food sources such as cereals, fruits and vegetables, to herbals as used in traditional medicine. The food or ingredient conferring health properties may consist of the plants themselves, extracts thereof, or more purified components. The scientific literature is abundant with articles not only on the beneficial properties, but also on possible adverse health effects of plants and their components. The present report discusses the data required to determine the safe use of these types of ingredients, and provides advice on the development of risk assessment strategies consistent with due diligence under existing food regulations. Product specifications, composition and characterisation of standardised and authentic materials, documented history of use and comparison to existing products (taking into account the effect of industrial processing), description of the intended use and consequent exposure are highlighted as key background information on which to base a risk evaluation. The extent of experimental investigation required, such as in vitro, animal, and/or human studies, depends on the adequacy of this information. A decision tree is presented as an aid to determine the extent of data requirements based on product comparison. The ultimate safety in use depends on the establishment of an adequate safety margin between expected exposure and identified potential hazards. Health hazards may arise from inherent toxicities or contaminants of the plant materials, including the mechanism of the intended beneficial effect. A lower safety margin may therefore be expected than for food ingredients or additives where no physiological effects are intended. In rare cases, post launch monitoring programmes may be envisaged to confirm expected exposures and adequacy of the safety margin. This guidance document was elaborated by an expert group of the Natural Toxin Task Force of the European Branch of the International Life Sciences Institute--ILSI Europe and discussed with a wider audience of scientists at a workshop held on 13-15 May 2002 in Marseille, France.

Coffee diterpenes prevent benzo[a]pyrene genotoxicity in rat and human culture systems

The coffee-specific diterpenes cafestol and kahweol (C+K) have been identified as two important chemoprotective agents in coffee. In the present study, the potential preventive effects of C+K against the genotoxicity of B[a]P were investigated in rat primary hepatocytes and in human bronchial Beas-2B cells. Several independent mechanisms were identified and their respective contribution to the overall protective effects was determined. A marked dose-dependent inhibition by C+K of B[a]P DNA-binding was found in cells of both origins. However, data showed that the significant induction by C+K of the detoxifying enzyme GST-Yp subunit is the key mechanism of protection against B[a]P DNA-binding in rat liver. In contrast, the phase I-mediated mechanism where C+K produce an inhibition of CYP 1A1 induction by B[a]P is of key significance for the C+K protection in human Beas-2B cells. Moreover, this effect suggests a novel mechanism of chemoprotection by the coffee diterpenes cafestol and kahweol.

Oligofructose does not affect the development of type 1 diabetes mellitus induced by dietary proteins in the diabetes-prone BB rat model

BACKGROUND

Prevention of Type 1 diabetes mellitus (T1DM), a major childhood chronic disease with rapidly increasing incidence, is an urgent topic of research. We investigated whether 5% oligofructose (OF) as compared to 5% cellulose had a protective effect against diet-induced T1DM in the diabetes-prone BioBreeding (BB) rat model.

METHODS

Groups of BB rats were fed the experimental diets from weaning. The diets were a cereal-based rodent diet (diabetogenic, positive control) and semi-synthetic rodent diets containing hydrolysed casein (non-diabetogenic, negative control), soy or whey as the sole protein source and 5% cellulose as fibre source. In additional groups fed soy and whey protein, the fibre source was 5% OF. T1DM incidence up to the age of 160 days was recorded applying biochemical and morphological criteria. Physiological effects of fibre were assessed through the analysis of biochemical parameters in plasma and of the protein/DNA ratio in intestinal mucosa.

RESULTS

T1DM incidence was diet-dependent. Cereal-, soy- and whey-based diets were significantly more diabetogenic than the hydrolysed casein-based diet. Five per cent OF did not affect the incidence of T1DM induced by either soy or whey proteins as compared to cellulose, nor induce any of the biological effects attributed to a fermentable fibre.

CONCLUSIONS

In the BB rat model, 5% OF in the diet did not have any protective effects against diet-induced T1DM. The present data do not suggest dietary OF as a promising approach for the dietary prevention of T1DM.

Cafestol and kahweol, two coffee specific diterpenes with anticarcinogenic activity

Epidemiological studies have found an inverse association between coffee consumption and the risk of certain types of cancers such as colorectal cancers. Animal data support such a chemopreventive effect of coffee. Substantial research has been devoted to the identification of coffee components that may be responsible for these beneficial effects. In animal models and cell culture systems, the coffee diterpenes cafestol and kahweol (C+K) were shown to produce a broad range of biochemical effects resulting in a reduction of the genotoxicity of several carcinogens including 7,12-dimethylbenz[a]anthracene (DMBA), aflatoxin B(1) (AFB(1)), benzo[a]pyrene (B[a]P) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Different mechanisms appear to be involved in these chemoprotective effects: an induction of conjugating enzymes (e.g. glutathione S-transferases, glucuronosyl S-transferases), an increased expression of proteins involved in cellular antioxidant defense (e.g. gamma-glutamyl cysteine synthetase and heme oxygenase-1) and an inhibition of the expression and/or activity of cytochromes P450 involved in carcinogen activation (e.g. CYP2C11, CYP3A2). In animal models, the C+K-mediated induction of conjugating and antioxidant enzymes has been observed in hepatic, intestinal and kidney tissues. In the small intestine, these inductions were shown to be mediated by Nrf2-dependent transcriptional activation. In vitro investigations obtained in cell cultures of human origin indicate that the effects and mechanisms observed in animal test systems with C+K are likely to be of relevance for humans. In human liver epithelial cell lines transfected to express AFB(1)-activating P450s, C+K treatment resulted in a reduction of AFB(1)-DNA binding. This protection was correlated with an induction of GST-mu, an enzyme known to be involved in AFB(1) detoxification. In addition, C+K was found to inhibit P450 2B6, one of the human enzymes responsible for AFB(1) activation. Altogether, the data on the biological effects of C+K provide a plausible hypothesis to explain some of the anticarcinogenic effects of coffee observed in human epidemiological studies and in animal experiments.

Protective effects of coffee diterpenes against aflatoxin B1-induced genotoxicity: mechanisms in rat and human cells

The coffee-specific diterpenes cafestol and kahweol (C + K) have been reported to be anticarcinogenic in several animal models. Proposed mechanisms involve a co-ordinated modulation of several enzymes responsible for carcinogen detoxification, thus preventing reactive agents interacting with critical target sites. To address the human relevance of the chemoprotective effects of C + K against aflatoxin B(1) (AFB1) genotoxicity observed in rat liver, and to compare the mechanisms of protection involved in both species, animal and human hepatic in vitro test systems were applied. In rat primary hepatocytes, C + K reduced the expression of cytochrome P450 CYP 2C11 and CYP 3A2, the key enzymes responsible for AFB1 activation to the genotoxic metabolite aflatoxin B1-8,9 epoxide (AFBO). In addition, these diterpenes induced significantly GST Yc2, the most efficient rat GST subunit involved in AFBO detoxification. These effects of C + K resulted in a marked dose-dependent inhibition of AFB1-DNA binding in this rat in vitro culture system. Their relevance in humans was addressed using liver epithelial cell lines (THLE) stably transfected to express AFB1 metabolising cytochrome P450s. In these cells, C + K also produced a significant inhibition of AFB1-DNA adducts formation linked with an induction of the human glutathione S-transferase GST-mu. Altogether, these results suggest that C + K may have chemoprotective activity against AFB1 genotoxicity in both rats and humans.

Oxidative damage and stress response from ochratoxin a exposure in rats

Ochratoxin A (OTA) is a mycotoxin found in some cereal and grain products. It is a potent renal carcinogen in male rats, although its mode of carcinogenic action is not known. Oxidative stress may play a role in OTA-induced toxicity and carcinogenicity. In this study, we measured several chemical and biological markers that are associated with oxidative stress response to determine if this process is involved in OTA-mediated toxicity in rats. Treatment of male rats with OTA (up to 2 mg/ 24 h exposure) did not increase the formation of biomarkers of oxidative damage such as the lipid peroxidation marker malondialdehyde in rat plasma, kidney, and liver, or the DNA damage marker 8-oxo-7,8-dihydro-2' deoxyguanosine in kidney DNA. However, OTA treatment (1 mg/kg) did result in a 22% decrease in alpha-tocopherol plasma levels and a 5-fold increase in the expression of the oxidative stress responsive protein haem oxygenase-1, specifically in the kidney. The selective alteration of these latter two markers indicates that OTA does evoke oxidative stress, which may contribute at least in part to OTA renal toxicity and carcinogenicity in rats during long-term exposure.

Activation of cytochrome P450 gene expression in the rat brain by phenobarbital-like inducers

Oxidative biotransformation, coupled with genetic variability in enzyme expression, has been the focus of hypotheses interrelating environmental and genetic factors in the etiology of central nervous system disease processes. Chemical modulation of cerebral cytochrome P450 (P450) monooxygenase expression character may be an important determinant of in situ metabolism, neuroendocrine homeostasis, and/or central nervous system toxicity resulting from exposure to neuroactive drugs and xenobiotic substances. To examine the capacity of the rat brain to undergo phenobarbital (PB)-mediated induction, we developed reverse transcription-polymerase chain reaction methods and evaluated the effects of several PB-like inducers on P450 and microsomal epoxide hydrolase gene expression. Animals treated i.p. with four daily doses of PB demonstrated markedly induced levels of CYP2B1, CYP2B2, and CYP3A1 mRNA in the striatum and cerebellum. In contrast, 1 or 2 days of PB treatment resulted in unchanged or even slightly decreased levels of CYP2B1 and CYP2B2 in the brain, although the latter treatments produced marked induction of the corresponding genes in the liver. Only slight increases in epoxide hydrolase RNA levels resulted in brains of PB-treated animals. Substantial activation of cerebral CYP2B1, CYP2B2, and CYP3A1 mRNA levels also resulted when animals were treated with the neuroactive drugs diphenylhydantoin and amitryptiline, and with the potential PB-like xenobiotic inducers trans-stilbene oxide and diallyl sulfide, whereas dichlorodiphenyltrichloroethane was less efficacious. Although the time course of the induction response is delayed in brain relative to that required for the liver, these results clearly establish that brain P450s are markedly PB inducible.

[Drug and psychological therapies in chronic subjective tinnitus. Meta-analysis of therapeutic effectiveness]

A metaanalysis on pharmacological and psychological treatment reports was conducted in order to evaluate the overall effectiveness of these treatment conditions in chronic tinnitus. A totality of 109 treatment studies published between 1976 and 3/1995 and meeting minimal inclusion criteria was located, but only 24 studies reporting sufficient information could be analyzed. The methodological quality of the studies was evaluated by a coding scheme, and the resulting index of effectiveness has been weighted by this standard as well as by sample size. Effect sizes of pharmacological studies were ES = 1.27 and by this higher than the ones for psychological studies with ES = 0.88. This finding is attenuated by high (unspecific) placebo effects of ES = 0.65 in pharmacological studies compared to ES = 0.10 in psychological studies. In general, patients from psychological studies further improve after treatment in cases where catamnestic measurements have been conducted.

Use of aggregating brain cell cultures to study developmental effects of organophosphorus insecticides

Aggregating brain cell cultures of fetal rat telencephalon can be grown in a chemically defined medium for extended periods of time. After a phase of intense mitotic activity, these three-dimensional cell cultures undergo extensive morphological differentiation, including synaptogenesis and myelination. To study the developmental toxicity of organophosphorus compounds (OP), aggregating brain cell cultures were treated with parathion. Protein content and cell type-specific enzyme activities were not affected up to a concentration of 10(5) M. Gliosis, characterized by an increased staining for glial fibrillary acidic protein (GFAP), was observed in immature and in differentiated cells. In contrast, uridine incorporation and myelin basic protein (MBP) immunoreactivity revealed strong differences in sensitivity between these two developmental stages. These results are in agreement with the view that in vivo the development-dependent toxicity is not only due to changes in hepatic detoxification, but also to age-related modifications in the susceptibility of the different populations of brain cells. Furthermore, they underline the usefulness of histotypic culture systems with a high developmental potential, such as aggregating brain cell cultures, and stress the importance of applying a large range of criteria for testing the developmental toxicity of potential neurotoxicants.

Maturation-dependent effects of chlorpyrifos and parathion and their oxygen analogs on acetylcholinesterase and neuronal and glial markers in aggregating brain cell cultures

An in vitro model, the aggregating brain cell culture of fetal rat telencephalon, has been used to study the maturation-dependent sensitivity of brain cells to two organophosphorus pesticides (OPs), chlorpyrifos and parathion, and to their oxon derivatives. Immature (DIV 5-15) or differentiated (DIV 25-35) brain cells were treated continuously for 10 days. Acetylcholinesterase (AChE) inhibitory potency for the OPs was compared to that of eserine (physostigmine), a reversible AChE inhibitor. Oxon derivatives were more potent AChE inhibitors than the parent compounds, and parathion was more potent than chlorpyrifos. No maturation-dependent differences for AChE inhibition were found for chlorpyrifos and eserine, whereas for parathion and paraoxon there was a tendency to be more effective in immature cultures, while the opposite was true for chlorpyrifos-oxon. Toxic effects, assessed by measuring protein content as an index of general cytotoxicity, and various enzyme activities as cell-type-specific neuronal and glial markers (ChAT and GAD, for cholinergic and GABAergic neurons, respectively, and GS and CNP, for astrocytes and oligodendrocytes, respectively) were only found at more than 70% of AChE inhibition. Immature compared to differentiated cholinergic neurons appeared to be more sensitive to OP treatments. The oxon derivates were found to be more toxic on neurons than the parent compounds, and chlorpyrifos was more toxic than parathion. Eserine was not neurotoxic. These results indicate that inhibition of AChE remains the most sensitive macromolecular target of OP exposure, since toxic effects were found at concentrations in which AChE was inhibited. Furthermore, the compound-specific reactions, the differential pattern of toxicity of OPs compared to eserine, and the higher sensitivity of immature brain cells suggest that the toxic effects and inhibition of AChE are unrelated.

Threshold of toxicological concern for chemical substances present in the diet: a practical tool for assessing the need for toxicity testing

The de minimis concept acknowledges a human exposure threshold value for chemicals below which there is no significant risk to human health. It is the underlying principle for the US Food and Drug Administration (FDA) regulation on substances used in food-contact articles. Further to this, the principle of Threshold of Toxicological Concern (TTC) has been developed and is now used by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) in their evaluations. Establishing an accepted TTC would benefit consumers, industry and regulators, since it would preclude extensive toxicity evaluations when human intakes are below such threshold, and direct considerable time and cost resources towards testing substances with the highest potential risk to human health. It was questioned, however, whether specific endpoints that may potentially give rise to low-dose effects would be covered by such threshold. In this review, the possibility of defining a TTC for chemical substances present in the diet was examined for general toxicity endpoints (including carcinogenicity), as well as for specific endpoints, namely neurotoxicity and developmental neurotoxicity, immunotoxicity and developmental toxicity. For each of these endpoints, a database of specific no-observed-effect levels (NOELs) was compiled by screening oral toxicity studies. The substances recorded in each specific database were selected on the basis of their demonstrated adverse effects. For the neurotoxicity and developmental neurotoxicity databases, it was intended to cover all classes of compounds reported to have either a demonstrated neurotoxic or developmentally neurotoxic effect, or at least, on a biochemical or pharmacological basis were considered to have a potential for displaying such effects. For the immunotoxicity endpoint, it was ensured that only immunotoxicants were included in the database by selecting most of the substances from the Luster et al. database, provided that they satisfied the criteria for immunotoxicity defined by Luster. For the developmental toxicity database, substances were selected from the Munro et al. database that contained the lowest NOELs retrieved from the literature for more than 600 compounds. After screening these, substances showing any effect which could point to developmental toxicity as broadly defined by the US were recorded in the database. Additionally, endocrine toxicity and allergenicity were addressed as two separate cases, using different approaches and methodology. The distributions of NOELs for the neurotoxicity, developmental neurotoxicity and developmental toxicity endpoints were compared with the distribution of NOELs for non-specific carcinogenic endpoints. As the immunotoxicity database was too limited to draw such a distribution of immune NOELs, the immunotoxicity endpoint was evaluated by comparing immune NOELs (or LOELs-lowest-observed-effect levels-when NOELs were not available) with non-immune NOELs (or LOELs), in order to compare the sensitivity of this endpoint with non-specific endpoints. A different methodology was adopted for the evaluation of the endocrine toxicity endpoint since data currently available do not permit the establishment of a clear causal link between endocrine active chemicals and adverse effects in humans. Therefore, this endpoint was analysed by estimating the human exposure to oestrogenic environmental chemicals and evaluating their potential impact on human health, based on their contribution to the overall exposure, and their estrogenic potency relative to endogenous hormones. The allergenicity endpoint was not analysed as such. It was addressed in a separate section because this issue is not relevant to the overall population but rather to subsets of susceptible individuals, and allergic risks are usually controlled by other means (i.e. labelling) than the Threshold of Toxicological Concern approach. (ABSTRACT TRUNCATED)

The application of in vitro data in the derivation of the acceptable daily intake of food additives

The acceptable daily intake (ADI) for food additives is commonly derived from the NOAEL (no-observed-adverse-effect level) in long-term animal in vivo studies. To derive an ADI a safety or uncertainty factor (commonly 100) is applied to the NOAEL in the most sensitive test species. The 100-fold safety factor is considered to be the product of both species and inter-individual differences in toxicokinetics and toxicodynamics. Although in vitro data have previously been considered during the risk assessment of food additives, they have generally had no direct influence on the calculation of ADI values. In this review 18 food additives are evaluated for the availability of in vitro toxicity data which might be used for the derivation of a specific data-derived uncertainty factor. For the majority of the food additives reviewed, additional in vitro tests have been conducted which supplement and support the short- and long-term in vivo toxicity studies. However, it was recognized that these in vitro studies could not be used in isolation to derive an ADI; only when sufficient in vivo mechanistic data are available can such information be used in a regulatory context. Additional short-term studies are proposed for the food additives which, if conducted, would provide data that could then be used for the calculation of data-derived uncertainty factors.

The naturally occurring food mycotoxin fumonisin B1 impairs myelin formation in aggregating brain cell culture

The effects of subchronical applications of the mycotoxin Fumonisin B1 (FB1) were analyzed in vitro, using aggregating cell cultures of fetal rat telencephalon as a model. As cells in the aggregates developed from an immature state to a highly differentiated state, with synapse and compact myelin formation, it was possible to study the effects of FB1 at different developmental stages. The results showed that FB1 did not cause cell loss and it had no effects on neurons. However it decreased strongly the total content of myelin basic protein, the main constituent of the myelin sheath, during the myelination period (DIV 18-28). The loss of myelin was not accompanied by a loss of oligodendrocytes, the myelinating cells. However FB1 had effects on the maturation of oligodendrocytes, as revealed by a decrease in the expression of galactocerebroside, and on the compaction of myelin, as shown by a reduction of the expression of the mnyelin/oligodendrocyte glycoprotein MOG. The content of the cytoskeletal component glial fibrillary acidic protein (GFAP) was decreased in differentiated astrocytes, exclusively, while neurons were not affected by 40 microM of FB1 applied continuously for 10 days. In summary, FB1 selectively affected glial cells. In particular, FB1 delayed oligodendrocyte development and impaired myelin formation and deposition.

Increased vulnerability of neurones and glial cells to low concentrations of methylmercury in a prooxidant situation

Using reaggregating rat brain cell cultures at two different stages of differentiation, we examined the biochemical effects of a 10-day treatment with nanomolar concentrations of methylmercuric chloride (monomethylmercury), in the presence or absence of promoters of hydroxyl radical formation (10 microM copper sulphate plus 100 microM ascorbate). A decrease in total protein content accounted for the general cytotoxicity of these compounds, whereas selective effects were assessed by determining the activities of cell type-specific enzymes. Methylmercury, up to 100 nM, as well as the copper ascorbate mixture, when applied separately, induced no general cytotoxicity, and only slight effects on neuronal parameters. However, when applying 100 nM methylmercury and the copper-ascorbate mixture together, a drastic decrease in neuronal and glial parameters was found. Under these conditions, the content of reactive oxygen species, assessed by 2',7'-dichlorofluorescin oxidation, increased greatly, while the activities of antioxidant enzymes decreased. In the presence of copper and ascorbate, differentiated cultures appeared more resistant than immature ones to low methylmercury concentrations (1-10 mM), but did undergo similar changes in both cell type-specific and antioxidant enzyme activities at 100 nM methylmercury. These results suggest that in prooxidant conditions low doses of mercury can become much more deleterious for the central nervous system.

The coffee-specific diterpenes cafestol and kahweol protect against aflatoxin B1-induced genotoxicity through a dual mechanism

The diterpenes cafestol and kahweol (C&K) have been identified in animal models as two potentially chemoprotective agents present in green and roasted coffee beans. It has been postulated that these compounds may act as blocking agents by producing a co-ordinated modulation of multiple enzymes involved in carcinogen detoxification. In this study, we investigated the effects of C&K against the covalent binding of aflatoxin B1 (AFB1) metabolites to DNA. Male Sprague-Dawley rats were treated with increasing amounts of a mixture of C&K in the diet (0-6200 p.p.m.) for 28 and 90 days. A dose-dependent inhibition of AFB1 DNA-binding was observed using S9 and microsomal subcellular fractions from C&K-treated rat liver in an in vitro binding assay. Significant inhibition was detected at 2300 p.p.m. and maximal reduction of DNA adduct formation to nearly 50% of the control value was achieved with 6200 p.p.m. of dietary C&K. Two complementary mechanisms may account for the chemopreventive action of cafestol and kahweol against aflatoxin B1 in rats. A decrease in the expression of the rat activating cytochrome P450s (CYP2C11 and CYP3A2) was observed, as well as a strong induction of the expression of the glutathione-S-transferase (GST) subunit GST Yc2, which is known to detoxify highly the most genotoxic metabolite of AFB1. These data and the previously demonstrated effects of C&K against the development of 7,12-dimethylbenz[a]anthracene (DMBA)-induced carcinogenesis at various tissue sites suggest the potential widespread effect of these coffee components against chemical carcinogenesis.

The ADI as a basis to establish standards for pesticide residues in food products for infants and children

Pesticides are widely used in agriculture to control a variety of detrimental organisms. As a consequence, low but measurable amounts of residues may be found in the food supply including food intended for infants and young children. This has been the cause of some alarm since it is difficult for the general public to understand the magnitude of health risk associated with the consumption of food contaminated with low levels of potentially toxic chemicals. In this context safety-based regulations for pesticide residues that ensure the protection of infants and young children are of crucial importance. In this article we discuss the applicability of the ADI to infants and children with regards to pesticides and outline a proposal which has been devised to establish residue limits for finished baby food products.

Effects of the naturally occurring food mycotoxin ochratoxin A on brain cells in culture

The potential of ochratoxin A (OTA) to damage brain cells was studied by using a three-dimensional cell culture system as model for the developing brain. Aggregating cell cultures of foetal rat telencephalon were tested either during an early developmental period, or during a phase of advanced maturation, over a wide range of OTA concentrations (0.4 nM to 50 microM). By monitoring changes in activities of cell type-specific enzymes (ChAt and GAD, for cholinergic and GABAergic neurones, respectively, GS for astrocytes and CNP for oligodendrocytes), the concentration-dependent toxicity and neurodevelopmental effects of OTA were determined. OTA proved to be highly toxic, since a 10-day treatment at 50 nM caused a general cytotoxicity in both mature and immature cultures. At 10 nM of OTA, cell type-specific effects were observed: in immature cultures, a loss in neuronal and oligodendroglial enzyme activities, and an increase in the activity of the astroglial marker glutamine synthetase were found, Furthermore, at 2 and 10 nM of OTA, a clustering of microglial cells was observed. In mature cultures, OTA was somewhat less potent, but caused a similar pattern of toxic effects. A 24 h-treatment with OTA resulted in a concentration-dependent decrease in protein synthesis, with IC50 values of 25 nM and 33 nM for immature and mature cultures respectively. Acute (24 h) treatment at high OTA concentrations (10 to 50 microM) caused a significant increase in reactive oxygen species formation, as measured by the intracellular oxidation of 2',7'-dichlorofluorescin. These results suggest that OTA has the potential to be a potent toxicant to brain cells, and that its effects at nanomolar concentrations are primarily due to the inhibition of protein synthesis, whereas ROS seem not to be involved in the toxicity mediated by a chronic exposure to OTA at such low concentrations.

Placental glutathione S-transferase (GST-P) induction as a potential mechanism for the anti-carcinogenic effect of the coffee-specific components cafestol and kahweol

The coffee specific diterpenes cafestol and kahweol (C + K) have been reported to be anti-carcinogenic in several animal models. It has been postulated that this activity may be related to their ability to induce glutathione S-transferases (GSTs). We investigated the influence of a mixture of C + K, incorporated at various levels in the diet of Sprague-Dawley rats, on the expression of different hepatic GST iso-enzymes. Liver samples were examined using isoform-specific GST substrates and antibodies, and highly selective oligomers were employed to determine effects at the RNA level. A dose-dependent increase in general GST activity was observed in male and female animals following 28 or 90 days of treatment. A time-course study demonstrated that the maximal effect was observed within 5 days of treatment. Little or no effect was found on the activity of GST alpha and mu iso-enzymes. The most striking observation was a dose-dependent induction of placental glutathione S-transferase (GST-P) which could be demonstrated at the mRNA, protein and enzymatic levels. This effect was observed in both male and female rats. The maximal induction was attained within 5 days of treatment with C + K, remained elevated with continued treatment, but was reversible on withdrawal of treatment. Immunohistochemical examination of liver slices revealed a strong even distribution of GST-P expression throughout the acinus at the highest dose of C + K, while at lower doses the induction of GST-P occurred predominantly in periportal hepatocytes. There was no indication of the presence of preneoplastic foci and, furthermore, the effect of C + K on the GST-P was completely reversible. These findings indicate that the anticarcinogenic mechanism of C + K may involve a specific induction of GST-P and suggest a potential role for GST-P in detoxifying carcinogenic compounds.

Limits for pesticide residues in infant foods: a safety-based proposal

A review of safety procedures for regulating pesticide residues in food commodities has demonstrated that maximum residue levels established by national and international bodies may not be suitable for direct application to finished infant products. We have developed a safety-based strategy specifically aimed at setting limits for pesticide residues in baby foods. The approach is based on the acceptable daily intake (ADI) together with the application, when necessary, of an additional uncertainty factor to account for the potential higher sensitivity of infants to toxicants. The need for this extra factor is dependent on the toxicological information available and may be particularly important for pesticides with a neurotoxic potential. Using this strategy we have evaluated safety-based residue limits for finished products based on a standard food intake pattern of a 4-month-old infant. For most of the pesticides examined the estimated limits fell within a range which can be controlled through existing quality assurance systems. This scientific approach appears usable in practice and is intended as a basis for stimulating discussions aimed at evaluating the need for new limits to ensure the optimal quality of infant products with respect to pesticide safety. This assessment demonstrates that for a number of pesticides there is a need for adequate developmental studies to provide assurance that the current ADI is appropriate for infants. A consequence may be the necessity for enhanced analytical sensitivity for the determination of pesticide residues in infant foods which would be compatible with limits based on an infant-adjusted ADI.

Regional distribution and expression modulation of cytochrome P-450 and epoxide hydrolase mRNAs in the rat brain

In the present study, we developed a very sensitive, semiquantitative assay based on the reverse transcriptase-coupled polymerase chain reaction to measure, in a region-selective manner, mRNA expression patterns within the brain for microsomal epoxide hydrolase and several cytochrome P-450s (P-450s) known to be induced by prototypic agents in other tissues. The P-450s assessed included the polyaromatic hydrocarbon-inducible CYP1A1 and CYP1A2 systems, together with the phenobarbital-inducible P-450s, CYP2B1, CYP2B2, CYP3A1, which were examined 18 hr after a single intraperitoneal dose of the respective inducing agents. Highly region-specific patterns of expression were evident for P-450 mRNAs within the rat brain. In the control, uninduced brain, CYP1A1 mRNAs were readily detected in the striatum and in the hypothalamus, and to a lesser extent in the other regions examined. The regional pattern of expression was similar for CYP1A2; however, a major difference was noted in the olfactory bulbs, characterized by a relatively high level of CYP1A2 mRNA but correspondingly low levels of CYP1A1. Within the brain regions examined, the highest content of CYP2B1 and CYP2B2 mRNAs were present in the striatum and in the cerebellum, whereas CYP3A1 levels varied only slightly across the respective regions. In contrast to the P-450s, microsomal epoxide hydrolase mRNAs were expressed at relative homogeneous amounts throughout the brain. beta-Naphthoflavone markedly increased the CYP1A1 and CYP1A2 mRNA contents of each brain region investigated, although this agent did not affect levels of epoxide hydrolase. At 18 hr post-treatment with phenobarbital, an optimal time period for hepatic induction, brain expression was characterized by a complex pattern of effects, with increased levels noted for CYP2B1 mRNA content in the medulla oblongata, midbrain, and cortex, but decreased contents measured in the cerebellum, the hypothalamus, and the striatum. In each of these respective regions, CYP2B2 content was profoundly decreased whereas epoxide hydrolase expression was slightly increased by the same treatment. These results establish that the central nervous system actively expresses a number of different biotransformation gene products in a regional specific and inducer-dependent manner, and suggest that for tissues exhibiting low regenerative capacity, like the brain, such reactions are likely to be of critical toxicological significance.

Gastro-oesophageal reflux in children: comparison of different durations, positions and sleep-awake periods of pH monitoring in the same patient

A group of 72 children (mean age: 21.7 months, range: 14 days-19 years) with symptoms of gastro-oesophageal reflux were investigated by 22 h pH monitoring. Using a Proxeda software, we compared, in the same patient, the specificity and sensitivity of pH monitoring during 3 h, 6 h, 12 h, 12 nocturnal hours and 3 postprandial hours, as well as the influence of position and the sleep and alert periods. Results showed that all the short pH monitorings were statistically less sensitive than 22 h pH monitoring (P < 0.025). As regards specificity, only the 12 nocturnal hours pH monitoring was not statistically different from the 22 h pH monitoring. Gastro-oesophageal reflux was more frequent when the patient was awake than during sleep. pH monitoring seemed more reliable in the recumbent than in the upright position. We conclude that long-term pH monitoring (22 h) is the test of choice to diagnose gastro-oesophageal reflux because it included sleep and alert periods as well as different positions.

Phase I and phase II xenobiotic reactions and metabolism of the food-borne carcinogen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline in aggregating liver cell cultures

Aggregating fetal liver cell cultures were tested for their ability to metabolize xenobiotics using ethoxycoumarin-O-deethylase (ECOD), as marker of phase I metabolism, and glutathione S-transferase (GST), as marker for phase II reactions. Significant basal activities, stable over 14 days in culture were measured for both ECOD and GST activities. The prototype cytochrome P450 inducers, 3-methylcholanthrene (3-MC) and phenobarbital (PB), increased ECOD and GST activities reaching an optimum 7 days after culturing, followed by a decline in activity. This decline was partially prevented by 1% dimethyl sulfoxide (DMSO) added chronically to the culture medium. DMSO was also found to induce ECOD activity and to a lesser extent GST activity. Furthermore, it potentiated in a dose-dependent manner the induction of ECOD by PB. The food-borne carcinogen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) is metabolically transformed through a number of pathways in vivo. It was therefore used to examine the metabolic capacity in fetal and adult liver cell aggregates. Metabolism of MeIQx was mainly through N2-conjugation, resulting in formation of the N2-glucuronide and sulfamate conjugates for non-induced fetal liver cells. These metabolites were also found in large amounts in non-induced adult liver cells. Low levels of cytochrome P450-mediated ring-hydroxylated metabolites were detected in both non-induced fetal and adult liver cells. After induction with arochlor (PCB) or 3-MC, the major pathway was ring-hydroxylation (cytochrome P450 dependent), followed by conjugation to beta-glucuronic or sulfuric acid. The presence of the glucuronide conjugate of N-hydroxy-MeIQx, a mutagenic metabolite, suggested an induction of P450 CYP1A2. The metabolism of MeIQx by liver cell aggregates is very similar to that observed in vivo and suggests that aggregating liver cell cultures are a useful model for in vitro metabolic studies in toxicology.

Effects of pseudorabies virus on the neuronal properties of PC12 cells

The effect of pseudorabies virus on neuronal functions was investigated in PC12 cells. During the period investigated, choline acetyltransferase was not affected, while the acetylcholinesterase activity declined steadily starting at 12 h post infection (p.i.), reaching its minimal level of 40% of the control value at 24 h p.i. In contrast, the activity of tyrosine hydroxylase, the key enzyme in catecholamine synthesis, increased to 150% of the control level by 15 h p.i., dropping off slowly with the appearance of viral cytopathology. In parallel, the infection induced, by a process independent of the extracellular Ca2+, an increased release of dopamine at 11 h p.i., followed by noradrenaline at 20 h p.i. In the infected cells, the intracellular content of catecholamine was maintained only in the presence of a high amount of catecholamine precursors in the culture medium. Three plaque-forming units per cell was the minimal multiplicity of infection required to obtain the maximal changes in enzyme activities; higher multiplicities induced more rapidly the maximal effects on tyrosine hydroxylase and acetylcholinesterase. Inhibition of DNA synthesis did not prevent the increase in tyrosine hydroxylase activity; however, protein synthesis was required. In conclusion, infection of the PC12 cells with pseudorabies virus induced significant changes in catecholaminergic and cholinergic metabolism, indicating the ability of this virus to interfere selectively with specialized neuronal functions.