Metabolism of ochratoxin A: absence of formation of genotoxic derivatives by human and rat enzymes

Ochratoxin A: Apoptosis and Aberrant Exit from Mitosis due to Perturbation of Microtubule Dynamics?

Ochratoxin A (OTA) is a potent nephrotoxin and causes high incidences of renal tumors in rodents. The molecular events leading to tumor formation by OTA are not well defined. Early pathological changes observed in kidneys of rats treated with OTA in vivo include frequent mitotic and abnormally enlarged cells, detachment of tubule cells, and apoptosis within the S3 segment of the proximal tubule, suggesting that OTA may interfere with molecules involved in the regulation of cell division and apoptosis. In this study, treatment of immortalized human kidney epithelial (IHKE) cells with OTA (0-50 microM) resulted in a time- and dose-dependent increase in apoptosis and activation of c-Jun N-terminal kinase. At the same time, OTA blocked metaphase/anaphase transition and led to the formation of aberrant mitotic figures and giant cells with abnormally enlarged and/or multiple nuclei, sometimes still connected by chromatin bridges. Immunostaining of the mitotic apparatus using an alpha-tubulin antibody revealed defects in spindle formation. In addition, OTA inhibited microtubule assembly in a concentration-dependent manner in a cell-free, in vitro assay. Interestingly, treatment with OTA also resulted in activation of the transcription factor nuclear factor kappa B (NFkappaB), which has recently been shown to promote cell survival during mitotic cell cycle arrest. Based on these observations, we hypothesize that the mechanism by which OTA promotes tumor formation involves interference with microtubuli dynamics and mitotic spindle formation, resulting in apoptosis or-in the presence of survival signals such as stimulation of the NFkappaB pathway-premature exit from mitosis. Aberrant exit from mitosis resulting in blocked or asymmetric cell division may favor the occurrence of cytogenetic abnormalities and may therefore play a critical role in renal tumor formation by OTA.

DNA adduct formation by ochratoxin A: review of the available evidence

The mycotoxin ochratoxin A (OTA) is a potent nephrotoxin and renal carcinogen in rodents. However, the mechanism of OTA-induced tumour formation is unknown and conflicting results regarding the potential of OTA to react with DNA have been obtained. While experiments using radiolabelled ((3)H or (14)C) OTA and liquid scintillation counting or accelerator mass spectrometry indicate lack of formation of covalent DNA-adducts, spots detected by (32)P-postlabelling have been attributed to treatment with OTA. However, these putative DNA-adducts have not been shown to contain OTA or part of the OTA molecule and so far no structural information has been provided. Consistent with the absence of DNA-binding of radiolabelled OTA, studies on biotransformation in vivo and in vitro indicate that OTA is poorly metabolized and does not form reactive intermediates capable of interacting with DNA. Recently however, the structures of a carbon- and an oxygen-bonded OTA-deoxyguanosine adduct which is formed by photoirradiation of OTA in the presence of deoxyguanosine have been reported and suggested to be involved in OTA carcinogenicity. The aim of this manuscript is to provide an overview of the available literature regarding DNA adduct formation by OTA.

Further arguments in favour of direct covalent binding of Ochratoxin A (OTA) after metabolic biotransformation

Ochratoxin A (OTA) is nephrotoxic to all animal species, carcinogenic for rats and mice and probably implicated in human Balkan endemic nephropathy and the associated urothelial tract tumour. Controversial results concerning genotoxicity and biotransformation of OTA have been generated. By (32)P post-labelling technique, a dose- and time-dependent DNA adduct formation is observed in vivo and in vitro. Use of several inducers or inhibitors of biotransforming enzymes (including cytochrome P 450, cyclooxygenase, lipoxygenase, glutathione-S-transferase), demonstrated that OTA is biotransformed into genotoxic derivatives damaging for DNA. Authentic C8dG-OTA standards have been synthesized by photo-oxidation. Both of them (C-C8 & O-C8) co-migrate on TLC with two adducts formed by in vitro incubation of OTA in the presence of kidney microsomes, and in vivo in kidney of pig or rodent fed OTA as well as in kidney and bladder tumour of humans exposed to OTA. Several OTA metabolites have been isolated from tissues or cells treated by OTA. The open ring lactone (OP-OTA) and quinone OTA (OTQ) are genotoxic.

Ochratoxin A induces oxidative DNA damage in liver and kidney after oral dosing to rats

The nephrotoxic/carcinogenic mycotoxin ochratoxin A (OTA) occurs as a contaminant in food and feed and may be linked to human endemic Balkan nephropathy. The mechanism of OTA-derived carcinogenicity is still under debate, since reactive metabolites of OTA and DNA adducts have not been unambiguously identified. Oxidative DNA damage, however, has been observed in vitro after incubation of mammalian cells with OTA. In this study, we investigated whether OTA induces oxidative DNA damage in vivo as well. Male F344 rats were dosed with 0, 0.03, 0.1, 0.3 mg/kg bw per day OTA for 4 wk (gavage, 7 days/wk, five animals per dose group). Subsequently, oxidative DNA damage was determined in liver and kidney by the comet assay (single cell gel electrophoresis) with/without use of the repair enzyme formamido-pyrimidine-DNA-glycosylase (FPG). The administration of OTA had no effect on basic DNA damage (determined without FPG); however, OTA-mediated oxidative damage was detected with FPG treatment in kidney and liver DNA of all dose groups. Since the doses were in a range that had caused kidney tumors in a 2-year carcinogenicity study with rats, the oxidative DNA damage induced by OTA may help to explain its mechanism of carcinogenicity. For the selective induction of tumors in the kidney, increased oxidative stress in connection with severe cytotoxicity and increased cell proliferation might represent driving factors.

Metabolism-mediated Ochratoxin A genotoxicity in the single-cell gel electrophoresis (Comet) assay

The mycotoxin Ochratoxin A (OTA) is a contaminant of food and feed commodities in many countries. Long-term exposure to OTA in humans has been associated with an increased incidence of a progressive nephropathy (BEN). Bio-activation of OTA has been implicated in the OTA-mediated toxicity, although inconsistent results have been reported. The aim of the present study was to investigate the genotoxic potency of OTA and its metabolites in NIH/3T3 cells stably expressing the human cytochrome P450 iso-enzymes CYP2C9 and CYP3A4, by using the single-cell gel electrophoresis (SCGE/Comet) assay, which detects single strand DNA breaks. The obtained results confirm the hypothesis that biotransformation processes mediate OTA toxicity, and differences in response were observed in CYP2C9-hOR and CYP3A4-hOR expressing cells, respectively. Results showed that biotransformation of OTA increased the genotoxicity. Measurement of reactive oxygen species (ROS) production showed that the OTA-induced ROS production corresponded to the OTA-induced genotoxicity in the used NIH/3T3 cell lines.

Toxicokinetics and toxicodynamics of ochratoxin A, an update

Ochratoxin A (OTA) is a mycotoxin produced by fungi of two genera: Penicillium and Aspergillus. OTA has been shown to be nephrotoxic, hepatotoxic, teratogenic and immunotoxic to several species of animals and to cause kidney and liver tumours in mice and rats. Because of differences in the physiology of animal species, wide variations are seen in the toxicokinetic patterns of absorption, distribution and elimination of the toxin. Biotransformation of OTA has not been entirely elucidated. At present, data regarding OTA metabolism are controversial. Several metabolites have been characterized in vitro and/or in vivo, whereas other metabolites remain to be characterized. Several major mechanisms have been shown as involved in the toxicity of OTA: inhibition of protein synthesis, promotion of membrane peroxidation, disruption of calcium homeostasis, inhibition of mitochondrial respiration and DNA damage. The contribution of metabolites in OTA genotoxicity and carcinogenicity is still unclear. The genotoxic status of OTA is still controversial because contradictory results were obtained in various microbial and mammalian tests, notably regarding the formation of DNA adducts. More recent studies are focused on the OTA ability to disturb cellular signalling and regulation, to modulate physiological signals and thereby to influence cells viability and proliferation. The present paper offers an update on these different issues. In addition since humans and animals are likely to be simultaneously exposed to several mycotoxins, especially through their diet, the little information available on the combined effects of OTA and other mycotoxins has also been reviewed.

Ochratoxin a causes DNA damage and cytogenetic effects but no DNA adducts in rats

Ochratoxin A (OTA) is a potent nephrotoxin and renal carcinogen in rats, but the mechanism of OTA tumorigenicity is unknown. Ochratoxin A has been shown to be negative in many genetic toxicology test in vitro. However, the potential of OTA to induce genotoxic effects has not been investigated in male rats, the most sensitive species for OTA-induced tumor formation. In this study, male F344 rats were repeatedly administered OTA (0, 250, 500, 1000, and 2000 microg/kg of body wt) or the non-chlorinated analogue ochratoxin B (OTB; 2000 microg/kg of body wt) for 2 weeks (5 days/week), and DNA breakage was analyzed in target and nontarget tissues using the comet assay both in the absence and presence of formamidopyrimidine-DNA (Fpg) glycosylase. Potential DNA-adduct formation was also analyzed in the target organ kidney by 32P-postlabeling using two different solvent systems. DNA-strand breaks were evident in liver, kidney, and spleen of animals treated with OTA, and a similar degree of DNA damage was observed in rats treated with OTB, despite the lower toxicity of OTB. Moreover, the presence of DNA damage did not correlate with histopathological alterations, which were evident in the kidney but not in the liver. In liver and kidney, the extent of DNA damage was further enhanced in the presence of Fpg glycosylase, which is known to convert oxidative DNA damage into strand breaks, suggesting the presence of oxidative DNA damage. Oxidative DNA damage as a mechanism of OTA-dependent DNA damage is consistent with the absence of lipophilic DNA adducts as assessed by 32P-postlabeling analysis. No spots indicative of OTA-related DNA adducts were observed in kidney DNA extracted from OTA-treated animals by 32P-postlabeling analysis, despite the use of synthetic standard for postulated adducts. A small, but not significant, increase in the incidence of chromosomal aberrations (essentially chromatid and chromosome-type deletions) was observed in splenocytes from rats treated with OTA in vivo and subsequently cultured in vitro to express chromosomal damage. These aberrations are also compatible with oxidative DNA lesions since they are not typically caused by chemical carcinogens which form covalent DNA adducts. Together, with the lack of evidence for formation of lipophilic DNA adducts as assessed by postlabeling, these data suggest that OTA may cause genetic damage in both target and nontarget tissues independent of direct covalent binding to DNA.

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.

Ambident reactivity of phenoxyl radicals in DNA adduction

Phenols are a class of compounds that can create beneficial effects in vivo owing to their antioxidant properties (through radical scavenging), or they can display hazardous effects owing to their pro-oxidant properties. The mechanism by which phenols act as pro-oxidants stems from their one-electron oxidation into reactive phenoxyl radicals by peroxidase enzymes or redox-active transition metals. In the presence of thiols and molecular oxygen, these reactive phenoxyl radicals stimulate an oxidative stress and cause oxidative damage to biomolecules, which is proposed to contribute to the occurrence of cancer in peroxidase rich tissues. Recent results from our laboratory show that certain phenoxyl radicals can also react directly with the C-8 site of deoxyguanosine to afford oxygen and carbon bonded adducts. This reactivity is consistent with the ambident (oxygen vs. C attachment) electrophilicity of phenoxyl radicals coupled with the susceptibility of the C-8 site of deoxyguanosine to radical attachment. Given that formation of covalent DNA adducts is regarded as the initiation event in the carcinogenic process, C-8 deoxyguanosine adducts of phenolic toxins are expected to contribute greatly to peroxidase driven toxic effects of phenolic xenobiotics. The focus of this review is the role of phenoxyl radicals in direct reactions with DNA and the use of Brown σ+ values to predict their reactivity.Key words: DNA adduction, phenoxyl radicals, chlorophenols, ochratoxin A, deoxyguanosine.

Functional, Biochemical, and Pathological Effects of Repeated Oral Administration of Ochratoxin A to Rats

Ochratoxin A (OTA), a mycotoxin produced by several fungi of Aspergillus and Penicillium species, may contaminate agricultural products, resulting in chronic human exposure. In rats, OTA is a potent nephrotoxin, and repeated administration of OTA for 2 years to rats in doses up to 0.21 mg/kg of body wt resulted in high incidences of renal tumors arising from the proximal tubular epithelial cells. The mechanism of tumor formation by OTA in the kidney is not well-defined, and controversial results regarding mode of action have been published. The aim of this study was to characterize dose-dependent changes induced by OTA by application of clinical chemistry, biochemical markers, and toxicokinetics for a better conclusion on modes of action. Administration of OTA (0, 0.25, 0.5, 1, and 2 mg/kg of body wt) to male F344 rats (n = 3 per group) by oral gavage for 2 weeks resulted in a dose-dependent increase in OTA plasma concentrations and concentrations of OTA in both liver and kidney. Although oxidative stress has been implicated in OTA carcinogenicity, treatment with OTA did not induce overt lipid peroxidation or an increase in 8-oxo-7,8-dihydro-2'deoxyguanosine (8-OH-dG) in kidney. In the kidney, OTA-induced pathology was present at all dose levels administered, with a clear increase in severity related to dose. Pathology was restricted to the outer stripe of the outer medulla and consisted of disorganization of the tubule arrangement, frequent apoptotic cells, and abnormally enlarged nuclei scattered through the S3 tubules. Consistent with the histopathology, a dose-dependent increase in the expression of proliferating cell nuclear antigen (PCNA), indicative of cell proliferation, was observed in kidneys, but not in livers of treated animals. The most prominent change in the composition of urine induced by OTA analyzed by 1H NMR and principal component analysis consisted of a major increase in the excretion of trimethylamine N-oxide. However, typical changes observed with other proximal tubular toxins such as increased excretion of glucose were not observed at any of the doses administered. Similarly, treatment with OTA had no clear effects on clinical chemical parameters indicative of nephrotoxicity, although urinary volume was increased at the higher-dose groups. Taken together, the uncommon changes induced by OTA suggest that a unique mechanism may be involved in OTA nephrotoxicity and carcinogenicity.

Structurally related mycotoxins ochratoxin A, ochratoxin B, and citrinin differ in their genotoxic activities and in their mode of action in human-derived liver (HepG2) cells: implications for risk assessment

To elucidate the effects of three structurally related mycotoxins, namely, ochratoxin A (OTA), ochratoxin B (OTB), and citrinin (CIT), on human health, we investigated their acute toxic, mitogenic, and genotoxic effects in the human-derived liver cell line (HepG2). These compounds are found in moldy foods in endemic areas of nephropathy, which is associated with urinary tract cancers. In agreement with previous experiments, we found that OTA causes a dose-dependent induction of micronuclei (MN) and DNA migration in the single-cell gel electrophoresis (SCGE) assay, which was statistically significant at concentrations of > or =5 microg/ml. In contrast, OTB was devoid of genotoxic activity under identical conditions, but the compound caused pronounced inhibition of cell division even at doses lower than OTA (10 microg/ml). CIT caused an effect similar to that of OTA in MN assays (significant at dose levels of > or =2.5 microg/ml) but was negative in the SCGE test. All compounds failed to induce mutations in Salmonella/microsome assays in strains TA 98 and TA 100 after addition of HepG2-derived enzyme homogenate (S9-mix). By use of DNA-centromeric probes we found that induction of MN by OTA involves chromosome breaking effects (55-60% of the MN were centromere negative), whereas CIT-induced MN were predominantly centromere positive (78-82%). Our findings indicate that OTB is devoid of genotoxic activity in human-derived cells and therefore probably not a genotoxic carcinogen in humans. In contrast, CIT was an equally potent inducer of MN in HepG2 cells as OTA, but this effect is caused by a different mechanism, namely, aneuploidy. Furthermore, our data suggest that combined exposure to structurally related mycotoxins that cause DNA damage via completely different mechanisms may significantly increase the cancer risk of humans consuming moldy foods.

Biotransformation and nephrotoxicity of ochratoxin B in rats

Ochratoxin B (OTB), a secondary metabolite of Aspergillus ochraceus, is the nonchlorinated analogue of the mycotoxin ochratoxin A (OTA), which is one of the most potent renal carcinogens in rodents. Despite the closely related structure, OTB is considered to be of much lower toxicity. OTA is poorly metabolized and slowly eliminated, and this may play an important role in OTA toxicity, carcinogenicity, and organ specificity. Since little is known regarding biotransformation and renal toxicity of OTB, the aim of this study was to investigate biotransformation of OTB in rats and to characterize the nephrotoxicity and cytotoxicity of OTB. Male F344 rats were administered either a single dose of OTB (10 mg/kg bw) or repeated doses (2 mg/kg bw, 5 days/week for 2 weeks) and euthanized 72 h after the last dosing. In proximal tubule cells of animals treated with a single high dose of OTB, a slight increase in mitotic figures was observed, but no treatment-related changes were evident in clinical chemistry, in renal function, and histopathology after repeated administration. Excretion of OTB and metabolites in urine and feces was analyzed using both HPLC with fluorescence detection and LC-MS/MS. Ochratoxin beta, which results from cleavage of the peptide bond, was the major metabolite excreted in urine in addition to small amounts of 4-hydroxy-OTB. In total, 19% of the administered dose was recovered as OTB and ochratoxin beta in urine and feces within 72 h after a single dose. In contrast to OTA, no tissue-specific retention of OTB was evident after single and repeated administration. In LLC-PK1 cells, a renal cell culture system that retains much of the specific features of the proximal tubule, only minor differences in the extent of cytotoxicity of OTA and OTB were observed. At low concentrations (< 25 microM), treatment with OTA was slightly more toxic, whereas reduction in cell viability was similar at concentrations up to 100 microM. In summary, these data suggest that OTA and OTB have a similar potential to induce cytotoxicity in vitro, but large differences in their potential to induce nephrotoxicity in rodents. OTB is more extensively metabolized and more rapidly eliminated than OTA. The lack of specific retention of OTB in the kidneys and the differences in toxicokinetics may therefore provide an explanation for the lower toxicity of OTB.

Ochratoxin A-induced DNA damage in human fibroblast: protective effect of cyanidin 3-O-β-d-glucoside

Ochratoxin A (OTA), a mycotoxin produced by Aspergillus ochraceus and other moulds, has recently received growing attention because of its carcinogenic, teratogenic and nephrotoxic properties in both humans and farm animals. Nevertheless, with regard to the mechanism of toxicity, the data in the literature are inconclusive. The aim of our work was to verify in human fibroblasts treated with different OTA dosages the involvement of oxidative pathway in the damage mechanism of this mycotoxin and the possible protective effect exerted by cyanidin 3-O-beta-D-glucoside (C3G), an anthocyanin present in pigmented oranges, red wines, fruits and vegetables. The addition of OTA at 25 and 50 microM concentrations for 48 h determined only a slight but significant (P<.05) increase in radical oxygen species, whereas a substantial increase in their production was observed at longer exposure, in particular, when the fibroblasts were treated with 50 microM OTA for 72 h. Under the same experimental conditions, our data showed a significant (P<.05) increase in the rupture of cellular membrane and high damage to genomic DNA, evaluated by single-cell gel electrophoresis (comet assay), thus confirming the involvement of oxidative stress in the OTA genotoxicity in agreement with other studies. Diversely, mitochondrial functionality does not appear influenced by OTA treatment. C3G (0.125, 0.250 mM) added to the cells treated with 50 microM OTA significantly reduced free radical species production and prevented genomic DNA damage.

Molecular aspects of the transport and toxicity of ochratoxin a

Ochratoxins are a class of naturally occurring compounds produced by several fungi. The most toxic is ochratoxin A (OTA), and occurrence of some human nephropathies and tumors correlate with enhanced OTA exposure. In this Account, the following areas are examined: molecular details of the binding of OTA to human serum albumin (HSA), the influences of binding to HSA on the trans-port of OTA across epithelial cell membranes by organic anion transport proteins, the oxidative activation of OTA, and the formation of OTA adducts with biological molecules. These studies are beginning to provide a detailed chemical model for the trans-port, accumulation, and genotoxic and carcinogenic effects of OTA.

Chemically induced renal tubule tumors in the laboratory rat and mouse: review of the NCI/NTP database and categorization of renal carcinogens based on mechanistic information

The incidence of renal tubule carcinogenesis in male and female rats or mice with 69 chemicals from the 513 bioassays conducted to date by the NCI/NTP has been collated, the chemicals categorized, and the relationship between carcinogenesis and renal tubule hyperplasia and exacerbation of the spontaneous, age-related rodent disease chronic progressive nephropathy (CPN) examined. Where information on mechanism or mode of action exists, the chemicals have been categorized based on their ability to directly or indirectly interact with renal DNA, or on their activity via epigenetic pathways involving either direct or indirect cytotoxicity with regenerative hyperplasia, or exacerbation of CPN. Nine chemicals were identified as directly interacting with DNA, with six of these producing renal tubule tumors at high incidence in rats of both sexes, and in some cases also in mice. Ochratoxin A was the most potent compound in this group, producing a high tumor incidence at very low doses, often with metastasis. Three chemicals were discussed in the context of indirect DNA damage mediated by an oxidative free radical mechanism, one of these being from the NTP database. A third category included four chemicals that had the potential to cause DNA damage following conjugation with glutathione and subsequent enzymatic activation to a reactive species, usually a thiol-containing entity. Two chemicals were allocated into the category involving a direct cytotoxic action on the renal tubule followed by sustained compensatory cell proliferation, while nine were included in a group where the cell loss and sustained increase in renal tubule cell turnover were dependent on lysosomal accumulation of the male rat-specific protein, alpha2mu-globulin. In a sixth category, morphologic evidence on two chemicals indicated that the renal tumors were a consequence of exacerbated CPN. For the remaining chemicals, there were no pertinent data enabling assignment to a mechanistic category. Accordingly, these chemicals, acting through an as yet unknown mechanism, were grouped as either being associated with an enhancement of CPN (category 7, 16 chemicals), or not associated with enhanced CPN (category 8, 4 chemicals). A ninth category dealt with 11 chemicals that were regarded as producing increases in renal tubule tumors that did not reach statistical significance. A 10th category discussed 6 chemicals that induced renal tumors in mice but not in rats, plus 8 chemicals that produced a low incidence of renal tubule tumors in mice that did not reach statistical significance. As more mechanistic data are generated, some chemicals will inevitably be placed in different groups, particularly those from categories 7 and 8. A large number of chemicals in the series exacerbated CPN, but those in category 7 especially may be candidates for inclusion in category 6 when further information is gleaned from the relevant NTP studies. Also, new data on specific chemicals will probably expand category 5 as cytotoxicity and cell regeneration are identified as obligatory steps in renal carcinogenesis in more cases. Additional confirmatory outcomes arising from this review are that metastases from renal tubule tumors, while encountered with chemicals causing DNA damage, are rare with those acting through an epigenetic pathway, with the exception being fumonisin B1; that male rats and mice are generally more susceptible than female rats and mice to chemical induction of renal tubule tumors; and that a background of atypical tubule hyperplasia is a useful indicator reflecting a chemically associated renal tubule tumor response. With respect to renal tubule tumors and human risk assessment, chemicals in categories 1 and 2, and possibly 3, would currently be judged by linear default methods; chemicals in category 4 (and probably some in category 3) as exhibiting a threshold of activity warranting the benchmark approach; and those in categories 5 and 6 as representing mechanisms that have no relevance for extrapolation to humans.

Metabolism-mediated cytotoxicity of ochratoxin A

Ochratoxin A (OTA) is produced by various strains of Aspergillus and Penicillium and is a common contaminant of food commodities. OTA is metabolised by cytochrome P450 (CYP450) enzymes resulting in hydroxylated metabolites, 4R-OH-OTA and 4S-OH-OTA, and possibly in other minor metabolites including OTA-quinones. However, until now conflicting data have been presented regarding the role of biotransformation products in the adverse effects of OTA. Hence, the aim of this study was to further assess the metabolism-mediated cytotoxicity of OTA in an in vitro model encompassing NIH/3T3 cells, stably expressing the human CYP450 enzymes CYP2C9 and CYP3A4, respectively. In addition, modulation of the cellular glutathione (GSH) content was used to identify a role of GSH in OTA-induced cytotoxicity. Following exposure to OTA, cells expressing CYP2C9 showed a significant reduction in neutral red (NR) uptake but not in Alamar blue (AB) reduction, as compared to the control LNCX cells which do not express CYP450 enzymes. CYP3A4-expressing cells showed no difference in viability from control LNCX cells. When pre-treated with l-buthionine S,R-sulphoximine (BSO) to deplete GSH, CYP2C9-expressing cells showed also a loss of cell viability as compared to LNCX cells, although to a lesser extent as compared to non-depleted CYP2C9-expressing cells. Data presented in this study support previous findings, indicating that different biotransformation pathways contribute to the cytotoxicity induced by OTA.

Failure of the standard battery of short-term tests in detecting some rodent and human genotoxic carcinogens

Theoretical reasons and experimental evidence indicate that a no-effect level generally cannot be expected for genotoxic carcinogens; as a consequence, in quantitative risk assessment the capability of distinguishing genotoxic from non-genotoxic carcinogens is of fundamental importance in order to identify relevant levels of human exposure. According to generally accepted guidelines, the standard three-test battery for the detection of genotoxic compounds consists of: (i) an in vitro test for gene mutation in bacteria; (ii) an in vitro test in mammalian cells with cytogenetic evaluation of chromosomal damage and/or a test that detects gene mutations; (iii) an in vivo test for chromosomal damage using rodent hematopoietic cells. This test battery is designed to avoid the risk of false negative results for compounds with genotoxic potential, but it cannot be taken for granted that the risk is completely eliminated. As a matter of fact there are some chemicals, classified by the International Agency for Research on Cancer (IARC) as probably or possibly carcinogenic to humans, which gave consistent negative results in this test battery, and in contrast provided positive results in other not routinely employed genotoxicity assays. The failure of the standard test battery in detecting some genotoxic carcinogens is attributable to several causes, but the principal of them are the following ones: in vitro, the artificial metabolic activity of the liver S9-mix, and the different biotransformation of chemicals in cells of different type and from different animal species; in vivo, the pharmacokinetic behaviour of the test compound, and its possible species-, sex- and tissue-specificity.

Ochratoxin A: Lack of Formation of Covalent DNA Adducts

The mycotoxin ochratoxin A (OTA) is a potent nephrotoxin and renal carcinogen in rodents. However, the mechanism of OTA-induced tumor formation is unknown and conflicting results have been obtained regarding the potential of OTA to bind to DNA. OTA is poorly metabolized, and no reactive intermediates capable of interacting with DNA have been detected in vitro or in vivo. Recently, a hydroquinone/quinone redox couple and a carbon-bonded OTA-deoxyguanosine (OTA-dG) adduct formed by electrochemical oxidation and photoreaction of OTA have been reported and suggested to be involved in OTA carcinogenicity. This study was designed to characterize the role of DNA binding and to determine if formation of these derivatives occurs in vivo and in relevant activation systems in vitro using specific and sensitive methods. Horseradish peroxidase activation of OTA and its dechlorinated analogue ochratoxin B (OTB) yielded ochratoxin A-hydroquinone (OTHQ), but the postulated OTA-dG adduct was not detectable using LC-MS/MS. In support of this, no OTA-related DNA adducts were observed by 32P-postlabeling. In vivo, only traces of OTHQ were found in the urine of male F344 rats treated with high doses of OTA (2 mg/kg body wt) for 2 weeks, suggesting that this metabolite is not formed to a relevant extent. In agreement with the in vitro data, OTA-dG was not detected by LC-MS/MS in liver and kidney DNA extracted from treated animals. In addition, DNA binding of OTA and OTB was assessed in male rats given a single dose of 14C-OTA or 14C-OTB using accelerator mass spectrometry, a highly sensitive method for quantifying extremely low concentrations of radiocarbon. The 14C content in liver and kidney DNA from treated animals was not significantly different from controls, indicating that OTA does not form covalent DNA adducts in high yields. In summary, the results presented here demonstrate that DNA binding of OTA is not detectable with sensitive analytical methods and is unlikely to represent a mechanism for OTA-induced tumor formation.

Toxicokinetics of the mycotoxin ochratoxin A in F 344 rats after oral administration

Ochratoxin A (OTA), a mycotoxin produced by several fungi of Aspergillus and Penicillium species, is a nephrotoxin and a renal carcinogen in rodents. This study was performed to investigate the biotransformation and toxicokinetics of this important food contaminant. Male (n=18) and female (n=18) F344 rats were administered a single dose of OTA (0.5 mg/kg b.w.) in corn oil by gavage. Animals (n=3) were sacrificed 24, 48, 72, 96, 672, and 1,344 hours after OTA administration and concentrations of OTA and OTA-metabolites in urine, feces, blood, liver, and kidney were determined by HPLC with fluorescence detection and/or by LC-MS/MS. Recovery of unchanged OTA in urine amounted to 2.1% of dose in males and 5.2% in females within 96 h. In feces, only 5.5% respectively 1.5% of dose were recovered. The major metabolite detected was OTalpha; low concentrations of OTA-glucosides were also present in urine. The maximal blood levels of OTA were observed between 24 and 48 h after administration and were appromixately 4.6 micromol/l in males and 6.0 micromol/l in females. Elimination of OTA from blood followed first-order kinetics with a half-life of approximately 230 h. In liver of both male and female rats, OTA-concentrations were less than 12 pmol/g tissue, with a maximum at 24 h after administration. In contrast, OTA accumulated in the kidneys, reaching a concentration of 480 pmol/g tissue in males 24 h after OTA-administration. Generally, tissue concentrations in males were higher than in females. OTalpha was not detected in liver and kidney tissue of rats administered OTA, and the OTalpha concentrations in blood were low (10-15 nmol/l). The high concentrations of OTA in kidneys of male rats may, in part, explain the organ- and gender-specific toxicity of OTA.

Stoichiometric preference in copper-promoted oxidative DNA damage by ochratoxin A

The ability of the fungal carcinogen, ochratoxin A (OTA, 1), to facilitate copper-promoted oxidative DNA damage has been assessed using supercoiled plasmid DNA (Form I)-agarose gel electrophoresis and gas chromatography-mass spectrometry with selected-ion monitoring (GC-MS-SIM). OTA is shown to promote oxidative cleavage of Form I DNA with optimal cleavage efficiency occurring under excess Cu(II) conditions. As the concentration of OTA was increased and present in excess of Cu(II) the cleavage was less effective. Parallel findings were found for the ability of the OTA-Cu mixture to facilitate oxidative base damage. Yields (lesions per 10(6) DNA bases) of modified bases upon exposure of calf-thymus DNA (CT-DNA) to OTA-H(2)O(2)-Cu(II) were diminished when the OTA:Cu ratio was increased to 5:1. Electrochemical studies carried out in methanol implicate a ligand-centered 2e oxidation of OTA in the presence of excess Cu(II), while product analyses utilizing electrospray mass spectrometry support the intermediacy of the quinone, OTQ (3), in Cu-promoted oxidation of OTA. The implications of these findings with regard to the mutagenicity of OTA are discussed.

Effects of the mycotoxin ochratoxin A in a bacterial and a mammalian in vitro mutagenicity test system

Ochratoxin A (OTA), a mycotoxin produced by several Aspergillus and Penicillium species, is a worldwide contaminant of food and feedstuffs. It is nephrotoxic, immunosuppressive and carcinogenic in several animal species. The mechanism by which OTA acts is not fully understood up to now. Here, OTA was evaluated for mutagenicity in the Salmonella typhimurium assay (Ames assay) and in the HPRT assay with V79 hamster fibroblasts. In the bacterial assay using the strains TA 98, TA 100, TA 1535, TA 1538, TA 102 and TA 104, OTA was not mutagenic at a concentration range from 0.01 to 500 micro M in the presence and absence of an external metabolising enzyme system (rat liver S9 enzyme mix). In V79 fibroblasts, cytotoxicity of OTA was estimated with the neutral red uptake assay. An IC(50) of 11.6 micro M was found in the absence and an IC(50) of 6.4 micro M in the presence of S9 mix. In the subsequent HPRT (hypoxanthine-guanine-phosphoribosyl-transferase) assay with V79 cells the negative result of the bacterial assay was confirmed using OTA in concentrations from 0.1 to 100 micro M. In order to obtain converted OTA metabolites from viable, metabolically competent cells, a preincubation of primary cultured rat hepatocytes with 0.016 to 0.8 micro M OTA was performed. The resulting culture medium, which contained OTA metabolites, was tested in both mutagenicity assays. Again, no mutagenic effect was detected either in the bacterial or in the mammalian test assay. In accordance with several literature data, the present results imply that OTA does not act as direct mutagen. Additionally, the OTA metabolites derived from cultured rat hepatocytes or rat liver S9 mix, also, do not have a mutagenic potency in the test systems used.

Photochemically catalyzed reaction of ochratoxin A with D- and L-cysteine

The photolysis (>300 nm) of ochratoxin A (OTA, N-[[(3R)-5-chloro-8-hydroxy-3-methyl-1-oxo-7-isochromanyl]carbonyl]-3-phenyl-L-alanine, 1) in the presence of excess (2 and 12 molar equiv) cysteine (CySH) has been investigated and found to yield sulfur adducts 5 and 6 that are characterized by liquid chromatography-mass spectrometry and 1H-NMR spectroscopy. The adduct 5 was ascribed to the Michael addition conjugate resulting from covalent attachment of CySH to the ochratoxin quinone (4) generated by photooxidation of OTA. This species was also formed by photolysis of a synthetic sample of the hydroquinone of OTA (ochratoxin hydroquinone, 3) in the presence of 12 equiv L-CySH. The conjugate 5 derived from photolysis of 3 with L-CySH was used for 1H-NMR analysis. The sulfur adduct 6 was the major species detected from covalent attachment of CySH to photoactivated OTA, and it resulted from direct displacement of the OTA Cl atom by CySH. The implications of the cysteinyl adducts to the in vivo toxicity of OTA are discussed, with particular emphasis given to conjugate 5, as products from the photooxidative pathway may be of relevance to the nephrotoxic properties of OTA.

The role of oxidative stress in the ochratoxin A-mediated toxicity in proximal tubular cells

Balkan endemic nephropathy (BEN), a disease characterized by progressive renal fibrosis in human patients, has been associated with exposure to ochratoxin A (OTA). This mycotoxin is a frequent contaminant of human and animal food products, and is toxic to all animal species tested. OTA predominantly affects the kidney and is known to accumulate in the proximal tubule (PT). The induction of oxidative stress is implicated in the toxicity of this mycotoxin. In the present study, primary rat PT cells and LLC-PK(1) cells, which express characteristics of the PT, were used to investigate the OTA-mediated oxidative stress response. OTA exposure of these cells resulted in a concentration-dependent elevation of reactive oxygen species (ROS) levels, depletion of cellular glutathione (GSH) levels and an increase in the formation of 8-oxoguanine. The OTA-induced ROS response was significantly reduced following treatment with alpha-tocopherol (TOCO). However, this chain-braking anti-oxidant did not reduce the cytotoxicity of OTA and was unable to prevent the depletion of total GSH levels in OTA-exposed cells. In contrast, pre-incubation of the cell with N-acetyl-L-cysteine (NAC) completely prevented the OTA-induced increase in ROS levels as well as the formation of 8-oxoguanine and completely protected against the cytotoxicity of OTA. In addition, NAC treatment also limited the GSH depletion in OTA-exposed PT- and LLC-PK(1) cells. From these data, we conclude that oxidative stress contributes to the tubular toxicity of OTA. Subsequently, cellular GSH levels play a pivotal role in limiting the short-term toxicity of this mycotoxin in renal tubular cells.

Balkan endemic nephropathy and associated urinary tract tumours: a review on aetiological causes and the potential role of mycotoxins

A series of publications in the 1950s described a kidney disease in Bulgaria, the former Yugoslavia and Romania that became known as Balkan endemic nephropathy (BEN). The disease was qualified by World Health Organisation (WHO) experts as 'progressive and very gradually developing renal failure with insidious onset.... The last stage shows marked fibrosis...'. BEN is characterized by tubular degeneration, interstitial fibrosis and hyalinization of glomeruli accompanied by enzymuria and impaired renal function without nephrotic syndrome. Later, an association between BEN and tumours of the kidney pelvis and ureter was recognized, so that the problem of BEN became not only nephrological, but also oncological. There may also be an association with increased urinary bladder cancer incidence, although many confounding factors may interfere in the analysis of data for this organ. In view of the very intimate association between BEN and the urinary tract tumours (UTT), the term 'endemic uropathy' has been proposed. Several hypothesis concerning the aetiology of these diseases has been investigated, which include: predisposing genes factors, environmental factors (heavy metals, minerals, bacteria, leptospira, viruses, fungal toxins and, most recently, pliocene lignites). This paper reviews the different hypotheses about the aetiology of endemic uropathy and pays particular attention to the role of fungal toxins.

Metabolism and lack of DNA reactivity of the mycotoxin ochratoxin a in cultured rat and human primary hepatocytes

It is still unclear whether the carcinogenic mycotoxin ochratoxin A (OTA) is bioactivated to DNA-binding metabolites in rodents and humans. Therefore, we have incubated cultured rat and human primary hepatocytes with noncytotoxic concentrations of (3)H-OTA ranging from 10(-7) to 10(-5) M for 8 h and determined its metabolism and covalent DNA binding. In rat hepatocytes, OTA was metabolized to small amounts of three products, which were further studied by electrospray ionization (ESI)-MS/MS techniques. In addition to 4-hydroxy-OTA, which is a known product of OTA biotransformation, two novel metabolites were detected and tentatively identified as hexose and pentose conjugates of OTA. The in vitro induction with 3-methylcholanthrene (3MC) increased the formation of 4-hydroxy-OTA but did not alter the formation of the conjugated metabolites. No covalent binding of (3)H-OTA or its metabolites to DNA was observed in rat hepatocytes with or without 3MC induction with a limit of detection of 2 adducts per 10(9) nucleotides. However, the cellular ratio of reduced glutathione to oxidized glutathione was significantly decreased by treatment with OTA. In cultured human hepatocytes, (3)H-OTA was only very poorly metabolized, and no covalent DNA binding was observed. In conclusion, the results of this in vitro study do not support the notion that OTA has the potential to undergo metabolic activation and form covalent DNA adducts in rodents and humans.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

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.