Factors that affect the occurrence of fumonisin

The two important Fusarium ear rots of corn, Gibberella ear rot (Fusarium graminearum, formally F. moniliforme and allied species) and Fusarium ear rot (F. verticillioides and allied species) grow under different environmental conditions. F. graminearum grows well only between 26 and 28 degrees C and requires rain both at silking and during disease progression. F. verticillioides grows well at higher temperatures, and ear rot and fumonisin accumulation are associated with drought and insect stress and growing hybrids outside their areas of adaptation. In southern Transkei, where esophageal cancer has been associated with the consumption of F. verticillioides and fumonisin-contaminated corn, environmental conditions favor this fungus in most years. In the nearby areas where the soils, crops, food consumption, and populations are the same and where esophageal cancer is low, temperatures are cooler and F. graminearum is favored. Although F. verticillioides is associated with a disease of corn, it may be that this fungus is a mutualistic endophyte of the plant. Perhaps because of this, breeding for resistance to Fusarium ear rot has produced inconclusive results to date. The best available strategies for reducing the risk of fumonisin contents of maize are to ensure that hybrids are adapted to the environment and to limit drought stress and insect herbivory. It may also be necessary to make use of alternative strategies such as producing hybrids that contain enzymes to degrade fumonisin as it is produced.

Biological control of Fusarium moniliforme in maize

Fusarium moniliforme Sheldon, a biological species of the mating populations within the (italic)Gibberella fujikuroi species complex, i.e., population A [= G. moniliformis (Sheld.) Wineland], is an example of a facultative fungal endophyte. During the biotrophic endophytic association with maize, as well as during saprophytic growth, F. moniliforme produces the fumonisins. The fungus is transmitted vertically and horizontally to the next generation of plants via clonal infection of seeds and plant debris. Horizontal infection is the manner by which this fungus is spread contagiously and through which infection occurs from the outside that can be reduced by application of certain fungicides. The endophytic phase is vertically transmitted. This type infection is important because it is not controlled by seed applications of fungicides, and it remains the reservoir from which infection and toxin biosynthesis takes place in each generation of plants. Thus, vertical transmission of this fungus is just as important as horizontal transmission. A biological control system using an endophytic bacterium, Bacillus subtilis, has been developed that shows great promise for reducing mycotoxin accumulation during the endophytic (vertical transmission) growth phase. Because this bacterium occupies the identical ecological niche within the plant, it is considered an ecological homologue to F. moniliforme, and the inhibitory mechanism, regardless of the mode of action, operates on the competitive exclusion principle. In addition to this bacterium, an isolate of a species of the fungus Trichoderma shows promise in the postharvest control of the growth and toxin accumulation from F. moniliforme on corn in storage.

Chromium(VI) Reduction by Catechol(amine)s Results in DNA Cleavage in Vitro: Relevance to Chromium Genotoxicity

Catechols are found extensively in nature both as essential biomolecules and as the byproducts of normal oxidative damage of amino acids and proteins. They are also present in cigarette smoke and other atmospheric pollutants. Here, the interactions of reactive species generated in Cr(VI)/catechol(amine) mixtures with plasmid DNA have been investigated to model a potential route to Cr(VI)-induced genotoxicity. Reduction of Cr(VI) by 3,4-dihydroxyphenylalanine (DOPA) (1), dopamine (2), or adrenaline (3) produces species that cause extensive DNA damage, but the products of similar reactions with catechol (4) or 4-tert-butylcatechol (5) do not damage DNA. The Cr(VI)/catechol(amine) reactions have been studied at low added H(2)O(2) concentrations, which lead to enhanced DNA cleavage with 1 and induce DNA cleavage with 4. The Cr(V) and organic intermediates generated by the reactions of Cr(VI) with 1 or 4 in the presence of H(2)O(2) were characterized by EPR spectroscopy. The detected signals were assigned to Cr(V)-catechol, Cr(V)-peroxo, and mixed Cr(V)-catechol-peroxo complexes. Oxygen consumption during the reactions of Cr(VI) with 1, 2, 4, and 5 was studied, and H(2)O(2) production was quantified. Reactions of Cr(VI) with 1 and 2, but not 4 and 5, consume considerable amounts of dissolved O(2), and give extensive H(2)O(2) production. Extents of oxygen consumption and H(2)O(2) production during the reaction of Cr(VI) with enzymatically generated 1 and N-acetyl-DOPA (from the reaction of Tyr and N-acetyl-Tyr with tyrosinase, respectively) were correlated with the DNA cleaving abilities of the products of these reactions. The reaction of Cr(VI) with enzymatically generated 1 produced significant amounts of H(2)O(2) and caused significant DNA damage, but the N-acetyl-DOPA did not. The extent of in vitro DNA damage is reduced considerably by treatment of the Cr(VI)/catechol(amine) mixtures with catalase, which shows that the DNA damage is H(2)O(2)-dependent and that the major reactive intermediates are likely to be Cr(V)-peroxo and mixed Cr(V)-catechol-peroxo complexes, rather than Cr(V)-catechol intermediates.

Exposure biomarkers in chemoprevention studies of liver cancer

Hepatocellular carcinoma (HCC) is the most common type of liver cancer, the major risk factors being hepatitis B and C viruses and aflatoxins; other factors such as alcohol are also of importance in some populations. Aflatoxin exposure biomarkers include urinary aflatoxin metabolites and aflatoxin-albumin adducts in peripheral blood. These biomarkers are well validated and have been applied in studies of many populations worldwide. They are proving to be valuable end-points in intervention studies, including chemoprevention studies. The biomarkers permit assessment of primary prevention measures to reduce aflatoxin intake. In addition, the determination of individual urinary aflatoxin metabolite profiles means that the effectiveness of chemopreventive agents designed to modulate aflatoxin metabolism can also be evaluated. Both aflatoxin-albumin adducts and urinary aflatoxin metabolites have been associated with increased HCC risk in prospective studies, indicating the predictive value of these biomarkers at the group level. However, given the multifactorial and multistep nature of HCC, it is unlikely that these exposure biomarkers will be predictive at the individual level or be of value as surrogate end-points in longer-term intervention trials aimed at reducing disease incidence. Aflatoxin-related mutations at codon 249 of the p53 gene in plasma may be more relevant in this regard but their application requires further understanding of the temporal appearance of this biomarker in relation to the natural history of the disease.

[Molecular and genetic epidemiology]

Molecular epidemiology is defined as "the use of biological markers in epidemiologic research" and genetic epidemiology is defined as "the study of the interaction between genetic and environmental factors in epidemiologic research". Traditional epidemiologic approaches defined as "the study of the distribution and determinants of disease frequency in human population" could not address the importance of genetic susceptibility of humans in disease occurrence. However, the use of biological or genetic markers identified and characterized by the help of advance in molecular biology and human genetics now can provide us better understanding of multi-factorial or multistep disease occurrence in humans. Biological markers used in molecular epidemiology are classified into three groups: biomarkers of exposure (i.e., carcinogen metabolites in human urine, DNA-adducts, etc.), biomarkers of effects (i.e., oncoproteins, tumor markers, etc.), and biomarkers of susceptibility (i.e., genetic polymorphisms of carcinogen metabolism enzymes, DNA repair, etc.). Susceptibility genes involved in disease pathogenesis are categorized into two groups: high penetrance genes (i.e., BRAC1, RB, etc.) and low penetrance genes (i.e., GSTs, XRCC1, etc.). This paper will address the usefulnesses of bomarkers in edpidemiologic research and will show the examples of the use of selected low penetrance genes involved in human carcinogenesis. The importance of multidisciplinary approaches among epidemiologists, molecular biologists, and human geneticists will also be discussed.

Evaluation of natural attenuation rate at a gasoline spill site

Contamination of groundwater by gasoline and other petroleum-derived hydrocarbons released from underground storage tanks (USTs) is a serious and widespread environmental problem. Natural attenuation is a passive remedial approach that depends upon natural processes to degrade and dissipate contaminants in soil and groundwater. Currently, in situ column technique, microcosm, and computer modeling have been applied for the natural attenuation rate calculation. However, the subsurface heterogeneity reduces the applicability of these techniques. In this study, a mass flux approach was used to calculate the contaminant mass reduction and field-scale decay rate at a gasoline spill site. The mass flux technique is a simplified mass balance procedure, which is accomplished using the differences in total contaminant mass flux across two cross-sections of the contaminant plume. The mass flux calculation shows that up to 87% of the dissolved total benzene, toluene, ethylbenzene, and xylene (BTEX) isomers removal was observed via natural attenuation at this site. The efficiency of natural biodegradation was evaluated by the in situ tracer method, and the first-order decay model was applied for the natural attenuation/biodegradation rate calculation. Results reveal that natural biodegradation was the major cause of the BTEX mass reduction among the natural attenuation processes, and approximately 88% of the BTEX removal was due to the natural biodegradation process. The calculated total BTEX first-order attenuation and biodegradation rates were 0.036 and 0.025% per day, respectively. Results suggest that the natural attenuation mechanisms can effectively contain the plume, and the mass flux method is useful in assessing the occurrence and efficiency of the natural attenuation process.

Diet, genetic susceptibility and carcinogenesis

At least six types of gene-environment interactions (GEI) have been proposed (Kouhry and Wagener, 1993). In the first type, neither the environmental exposure (EE) nor the genetic risk factor (GRF) have any effect by themselves, but interaction between them causes disease. This is the case of phenylalanine exposure and the phenylketonuria genotype. Type 2 is a situation in which the GRF has no effect on disease in the absence of exposure, but exacerbates the effects of the latter. This is the most important type of GEI in relation to metabolic susceptibility genes and human carcinogenesis. The third type is the converse of the second (EE is ineffective per se, but enhances the effect of GRF). Type 4 occurs when both FE and GRF increase the risk for disease, but the combination is interactive or synergistic: an example is the interaction between Xeroderma Pigmentosum and LWV radiation. Types 5 and 6, according to the classification proposed by Kouhry, refer to cases in which the GRF is protective. The model of GEI that is emerging as the most important in chemical carcinogenesis refers to metabolic susceptibility genes. The general population can be divided into subgroups depending on their susceptibility to the action of carcinogens, based on their ability to metabolize such compounds to electrophilic, reactive metabolites (which form adducts with DNA), or, respectively, electrophobic metabolites that are excreted. The present contribution is a short review of the relevant literature, with particular emphasis on some polymorphisms involved in dietary exposures. In addition, the practical implications of genetic testing in this field are discussed.

Tissue sphinganine as a biomarker of fumonisin-induced apoptosis

NCTR measured sphinganine concentrations in the livers of mice and in the livers and kidneys of rats in conjunction with a tumour bioassay. In our model of the tumour incidence, target-tissue levels of sphinganine serve as a biomarker for a dose response of fumonisin B1 on cell death. Initially we questioned the utility of sphinganine levels in this role because they were highly variable when compared across time points. In spite of this concern, a conceptual framework and data are presented that support the use of sphinganine as a biomarker for a dose response of fumonisin B1 on cell death. This framework is reasonably consistent with observed sphinganine concentrations in the examined tissues, the literature on fumonisin's effects on sphingolipid synthesis, and our hypothesized mechanism through which fumonisin B1 increases age-specific tumour incidence.

[In vitro study of the interference of certain food components with the metabolism of aflatoxin B1]

Some compounds naturally present in food (quercetin, beta-naphthoflavone), used as food additives (butylated hydroxytoluene, sodium sulfite) or resulting from the way they were cooked (2-aminodipyrido [1,2-a; 3', 2'-d] imidazole, norharmane) can interfere with AFB1 metabolism. These interferences have been studied in vitro by evaluating the production of adducts to glutathione and by the Ames test on Salmonella typhimurium. Whereas all compounds produced a drastic decrease of the mutagenic activity, the first three only (quercetin, beta-naphthoflavone, butylated hydroxytoluene) interfered with the production of the adducts to glutathione.

Non-oxidative mechanisms are responsible for the induction of mutagenesis by reduction of Cr(VI) with cysteine: role of ternary DNA adducts in Cr(III)-dependent mutagenesis

Intracellular reduction of carcinogenic Cr(VI) generates Cr-DNA adducts formed through the coordination of Cr(III) to DNA phosphates (phosphotriester-type adduct). Here, we examined the role of Cr(III)-DNA adducts in mutagenesis induced by metabolism of Cr(VI) with cysteine. Reduction of Cr(VI) caused a strong oxidation of 2', 7'-dichlorofluoroscin (DCFH) and extensive Cr-DNA binding but no DNA breakage. Cr-DNA adducts induced unwinding of supercoiled plasmids and structural distortions in the DNA helix as detected by decreased ethidium bromide binding. Propagation of Cr-treated pSP189 plasmids in human fibroblasts led to a dose-dependent formation of the supF mutants and inhibition of replication. Blocking of Cr(III)-DNA binding by occupation of DNA phosphates with Mg(2+) or by sequestration of Cr(III) by inorganic phosphate or EDTA eliminated mutagenic responses and restored a normal yield of replicated plasmids. Dissociation of Cr(III) from DNA by a phosphate-based reversal procedure returned mutation frequency to background levels. The mutagenic responses at the different phases of the reduction reaction were unrelated to the amount of reduced Cr(VI) but reflected the number and the spectrum of Cr(III)-DNA adducts that were formed. Ternary cysteine-Cr(III)-DNA adducts were approximately 4-5 times more mutagenic than binary Cr(III)-DNA adducts. Although intermediate reaction products (CrV/IV, thiyl radicals) were capable of oxidizing DCFH, they were insufficiently reactive to damage DNA. Single-base substitutions at G/C pairs were the predominant type of Cr-induced mutations. The majority of mutations occurred at the sites where G had adjacent purine in the 3' or 5' position. Overall, our results present the first evidence that Cr(III)-DNA adducts play the dominant role in the mutagenicity caused by the metabolism of Cr(VI) by a biological reducing agent.

Quaternized wood as sorbent for hexavalent chromium

The potential of quaternized wood (QW) chips in removing hexavalent chromium from synthetic solution and chrome waste under both batch and continuous-flow conditions was investigated. Sorption was found to be dependent on pH, metal concentration, and temperature. QW chips provide higher sorption capacity and wider pH range compared with untreated wood chips. The equilibrium data could be fitted into the Langmuir isotherm model, and maximum sorption capacities were calculated to be 27.03 and 25.77 mg/g in synthetic chromate solution and chrome waste, respectively. The presence of sulfate in high concentration appeared to suppress the uptake of chromium by QW chips. Column studies showed that bed depth influenced the breakthrough time greatly whereas flow rate of influent had little effect on its sorption on the column.

Effect of compost in phytoremediation of diesel-contaminated soils

The effect of compost on phytoremediation of diesel-contaminated soils was investigated using 130 small (200 g) containers in two screening tests. The experiments were conducted in a controlled environment using ryegrass from seed. Containers were destructively sampled at various times and analyzed for plant mass and total petroleum hydrocarbons. The results indicate that the presence of diesel reduces grass growth, and that compost helps reduced the impact of diesel on grass growth. The addition of compost helps increase diesel loss from the soils both with and without grass, though the addition of grass leads to lower diesel levels compared with controls. A second set of experiments indicates that the compost helps in phytoremediation of diesel-contaminated soil independent of the dilution effect that compost addition has. The results indicate that the compost addition allowed diesel loss down to 200 mg TPH/kg even though the compost would be expected to hold the diesel more tightly in the soil/compost mixture. The simplicity of the screening tests led to difficulties in controlling moisture content and germination rates. The conclusion of the research is that the tilling of compost into soils combined with grass seeding appears to be a valuable option for treating petroleum-contaminated soils.

Effects of human intestinal flora on mutagenicity of and DNA adduct formation from food and environmental mutagens

Although the intestinal flora is believed to have a critical role in carcinogenesis, little is known about the role of the human intestinal flora on the effects of mutagens in vivo. The aim of the present study was to address a possible role of the human intestinal flora in carcinogenesis, by exploiting human-flora-associated (HFA) mice. The capacity of human faeces to activate or inactivate 2-amino-3-methyl-3H:-imidazo[4,5-f]quinoline (IQ) and 2-nitrofluorene was determined using the Ames assay. Human faecal suspensions that were active in this regard were then selected and orally inoculated into germfree NMRI mice to generate HFA mice. HFA, germfree, conventionalized and conventional mice were administered IQ, 2-amino-9H:-pyrido[2,3-b]indole (2-amino-alpha-carboline; AAC) and 2-nitrofluorene. The activity of human intestinal flora against mutagens could be transferred into the mice. In comparing germfree mice and mice harbouring an intestinal flora, the presence of a flora was essential for the activities of faeces against mutagens. After administration of IQ and 2-nitrofluorene, DNA adducts were observed in the mice with a flora, while adducts were extremely low or absent in germfree animals. DNA adducts after AAC treatment were higher in germfree mice in some tissues including colon than in mice with bacteria. Differences in DNA adduct formation were also observed between HFA mice and mice with mouse flora in many tissues. These results clearly indicate that the intestinal flora have an active role in DNA adduct formation and that the role is different for the different chemicals to which the animals are exposed. The results also demonstrate that the human intestinal flora have different effects from the mouse flora on DNA adduct formation as well as in vitro metabolic activities against mutagens. Studies using HFA mice could thus provide much-needed information on the role of the human intestinal flora on carcinogenesis in vivo.

[Activation of environmental carcinogenic N-nitrosodialkylamines by model systems for metabolic oxidation]

Nitrosamines are environmental carcinogens, and their relevance to human cancer is highly suspected. Dialkylnitrosamines require activation through metabolizing enzymes before they become ultimate electrophilic active species. This review summarizes two model systems for metabolic oxidation of dialkylnitrosamines. One system utilizes porphyrin and oxidant as a model for shunt pathway in the metabolizing pathway of cytochrome P450, and the other one utilizes Fenton reagent. Porphyrin and oxidant activated nitrosodialkylamines into direct acting mutagen by releasing aldehydes. During the process the alkylating activity was observed and alcohols are formed from the alkylation of water. Fenton reagent, consisting of iron salt and hydrogen peroxide supplemented with copper salt, activated dialkylnitrosamines into mutagens of a novel type containing an oxadiazine ring as their proposed structures. These works with model systems open a new aspect into the elucidation of the mechanism of xenobiotic metabolism.

In vitro metabolism of fumonisin B1 by ruminal microflora

Fumonisin B1 (FB1) is a mycotoxin produced by Fusarium moniliforme and F. proliferatum. Little is known of its metabolic fate after oral ingestion in ruminants, but these animals are reported to be tolerant towards FB1. The metabolism of this mycotoxin was evaluated following incubation (1 microg/ml) in ruminal fluid for up to 72 h, in the presence or absence of alfalfa as a substrate for microbial growth, using a model rumen (sealed flask, anaerobic conditions, exclusion of light, gentle agitation, 39 degrees C). The decrease in FB1 concentration and the production of short-chain fatty acids were determined. FB1 had no effect on SCFA production. After 72 h incubation, FB1 depletion was 12% and 18% in samples with and without alfalfa, respectively. No hydrolysed metabolites (aminopolyols or aminopentol) were detected. These results indicate that FB1 is poorly metabolized in the rumen and suggest that such metabolism is not the cause of the tolerance to this toxin displayed by ruminants.

Cytochrome P450-2E1 and glutathione S-transferase mu polymorphisms among Caucasians and mulattoes from Brazil

The variable interindividual ability to metabolize environmental toxicants, also known as metabolic polymorphism, may be of substantial importance in the modulation of cancer risk. The ethnic distribution of these polymorphisms could be interesting in order to establish an association with cancer risk or even to establish selective advantage of some genotypes. Cytochrome P450 2E1 (CYP2E1) is a secondary enzyme that can metabolize ethanol, and glutathione S-transferase (GSTM1) is thought to be involved in the detoxification of epoxides and polycyclic aromatic hydrocarbons. Mutation in these genes was investigated in a random sample of healthy subjects from São Paulo, Brazil, which included 206 Caucasians and 86 mulattoes. Pst I restriction fragment length polymorphism (RFLP) in the 5'-flanking region of the CYP2E1 gene has been identified in 10.2% of Caucasian individuals and in 11.6% of mulattoes. For GSTM1 the frequency of the null genotype was significantly higher in Caucasian individuals (60.2%) than in mulattoes (41.9%). Allele frequencies were (1) CYP2E1 locus: P = 0.949, q = 0.051, se(p) = se(q) = 0.011 among Caucasians; and p = 0.942; q = 0.058; se(P) = se'(q) = 0.018 among mulattoes; and (2) GSTM1 locus: p = 0.224, q = 0.776, se(p) = se(q) = 0.022 among Caucasians; and p = 0.353; q = 0.647; se(p) = se(q) = 0.041 among mulattoes.

Identification of the human liver microsomal cytochrome P450s involved in the metabolism of N-nitrosodi-n-propylamine

The ability of human liver cytochrome P450s to metabolize the environmental carcinogen N-nitrosodi-n-propylamine (NDPA) was investigated. The maximum rate of NDPA depropylation in seven human liver microsomal samples was 1.15 nmol/min/mg (range 0.53-2.60). Troleandomycin, a P450 3A4/5 inhibitor, inhibited depropylation modestly (10-60%) in three of seven samples. Diethyldithiocarbamic acid, a potent 2E1 inhibitor, and a 2E1 inhibitory monoclonal antibody (mAb) inhibited the reaction in all samples (23 to almost 100%). No significant inhibition was observed with the 2C9 inhibitor sulfaphenazole or with mAbs to 3A4, 2A6 and 2D6. The 2C8/9/18/19 mAb inhibited depropylation in one sample by approximately 25% and approximately 25% of the activity in another sample could not be accounted for by the inhibitors. Denitrosation of NDPA by three of the microsomal samples exhibited low K(m) values (51-86 microM) while two of these also had high K(m) values (2.6 and 4.6 mM). Purified human P450 2B6 and 3A4 and human P450 2A6, 2C8, 2C9 and 2D6 membranes had high K(m) values relative to their maximum turnover rates and are unlikely to participate in NDPA metabolism at micromolar concentrations. Conversely, purified rabbit 2E1 exhibited K(m) and V(max) values for depropylation of 52 microM and 13.4 nmol propionaldehyde/min/nmol P450, respectively. Values for denitrosation were 66 microM and 1.44 nmol nitrite/min/nmol P450, respectively. The toxicity of NDPA in transfected human liver epithelial cells expressing 2E1 was dose dependent down to 50 microM. No toxicity was observed in control cells or those expressing 2A6. These results indicate that 2E1 is the major human liver microsomal isoform responsible for NDPA metabolism at low micromolar concentrations. We also show that purified P450s catalyze the denitrosation of NDPA at approximately 10-20% of the rate of depropylation and K(m) values for both reactions are the same for each isozyme. This is consistent with the formation of an initial intermediate common to both pathways, presumably an alpha-nitrosamino radical.

Lipid peroxidation, antioxidant enzymes, and benzo[a]pyrene-quinones in the blood of rats treated with benzo[a]pyrene

The lipid peroxidation (as malondialdehyde, MDA), activities of superoxide dismutase (SOD) and catalase (CAT), and benzo[a]pyrene (BaP) metabolites were investigated in sera and erythrocytes of male Sprague-Dawley rats treated with BaP (20 mg per rat). MDA levels were significantly increased in sera (16.98+/-3.29 nmol/ml serum, P<0.05) 12 h after BaP treatment and persisted up to 96 h (13.80+/-1. 65 nmol/ml serum, P<0.05), but no significant change in NIDA levels was observed in erythrocytes. SOD and CAT activities were significantly increased in erythrocytes shortly after BaP exposure, and they were slightly decreased in sera, indicating an inverse correlation between lipid peroxidation and antioxidant enzyme activity. BaP and BaP-quinones (BaP-1,6-quinone and BaP-3,6-quinone) were measured in sera during the study period. A rapid increase of unmetabolized BaP was observed in sera (41.27+/-4.14 pmol/ml serum) 3 h after BaP treatment, reaching a peak at 6 h (48.56+/-4.62 pmol/ml serum) followed by a sharp decrease. Formation of the BaP-1, 6-quinone and BaP-3,6-quinone started in sera 3 h after BaP treatment, reached a peak at 24 h (7.23+/-1.02 pmol/ml serum) and 12 h (9.20+/-0.98 pmol/ml serum), respectively, and then decreased gradually. The time-dependent pattern of serum lipid peroxidation and the level of erythrocyte antioxidant enzymes were shown to be related to the concentrations of the BaP-quinone metabolites. These results suggest that BaP treatment, probably via the formation of BaP-quinones, oxidatively altered lipids and antioxidant enzymes in the blood, and might be associated with BaP-related vascular toxicity including carcinogenesis.

Mutagenicity of trichloroethylene and its metabolites: implications for the risk assessment of trichloroethylene

This article addresses the evidence that trichloroethylene (TCE) or its metabolites might mediate tumor formation via a mutagenic mode of action. We review and draw conclusions from the published mutagenicity and genotoxicity information for TCE and its metabolites, chloral hydrate (CH), dichloroacetic acid (DCA), trichloroacetic acid (TCA), trichloroethanol, S-(1, 2-dichlorovinyl)-l-cysteine (DCVC), and S-(1, 2-dichlorovinyl) glutathione (DCVG). The new U.S. Environmental Protection Agency proposed Cancer Risk Assessment Guidelines provide for an assessment of the key events involved in the development of specific tumors. Consistent with this thinking, we provide a new and general strategy for interpreting genotoxicity data that goes beyond a simple determination that the chemical is or is not genotoxic. For TCE, we conclude that the weight of the evidence argues that chemically induced mutation is unlikely to be a key event in the induction of human tumors that might be caused by TCE itself (as the parent compound) and its metabolites, CH, DCA, and TCA. This conclusion derives primarily from the fact that these chemicals require very high doses to be genotoxic. There is not enough information to draw any conclusions for trichloroethanol and the two trichloroethylene conjugates, DCVC and DCVG. There is some evidence that DCVC is a more potent mutagen than CH, DCA, or TCA. Unfortunately, definitive conclusions as to whether TCE will induce tumors in humans via a mutagenic mode of action cannot be drawn from the available information. More research, including the development and use of new techniques, is required before it is possible to make a definitive assessment as to whether chemically induced mutation is a key event in any human tumors resulting from exposure to TCE.

Pulmonary toxicity and carcinogenicity of trichloroethylene: species differences and modes of action

Trichloroethylene (TCE) is both acutely toxic and carcinogenic to the mouse lung following exposure by inhalation. In contrast, it is not carcinogenic in the rat lung and is markedly less toxic following acute exposure. Toxicity to the mouse lung is confined almost exclusively to the nonciliated Clara cell and is characterized by vacuolation and increases in cell replication. Chloral, a metabolite of TCE that accumulates in Clara cells and has been shown to be the cause of the toxicity, also causes aneuploidy in some test systems. Cytotoxicity, increased cell division, and aneuploidy are known risk factors in the development of cancer and provide a plausible mode of action for TCE as a mouse lung carcinogen. All acute and chronic effects of TCE on the mouse lung are believed to be a direct consequence of high cytochrome P450 activity and impaired metabolism of chloral in Clara cells. Comparisons between species suggest that the ability of the human lung to metabolize TCE is approximately 600-fold less than that in the mouse. In addition, the human lung differs markedly from the mouse lung in the number and morphology of its Clara cells. Thus, the large quantitative differences between the metabolic capacity of the mouse lung and the human lung, together with the species differences in the number and morphology of lung Clara cells, suggest that the risks to humans are minimal and that other tumor sites should take precedent over the lung when assessing the potential risks to humans exposed to TCE.

Human variability and susceptibility to trichloroethylene

Although humans vary in their response to chemicals, comprehensive measures of susceptibility have generally not been incorporated into human risk assessment. The U.S. EPA dose-response-based risk assessments for cancer and the RfD/RfC (reference dose-reference concentration) approach for noncancer risk assessments are assumed to protect vulnerable human subgroups. However, these approaches generally rely on default assumptions and do not consider the specific biological basis for potential susceptibility to a given toxicant. In an effort to focus more explicitly on this issue, this article addresses biological factors that may affect human variability and susceptibility to trichloroethylene (TCE), a widely used halogenated industrial solvent. In response to Executive Order 13045, which requires federal agencies to make protection of children a high priority in implementing their policies and to take special risks to children into account when developing standards, this article examines factors that may affect risk of exposure to TCE in children. The influence of genetics, sex, altered health state, coexposure to alcohol, and enzyme induction on TCE toxicity are also examined.

Modes of action of trichloroethylene for kidney tumorigenesis

This article focuses on the various models for kidney toxicity due to trichloroethylene (TCE) and its glutathione-dependent metabolites, in particular S-(1,2-dichlorovinyl)-l-cysteine. Areas of controversy regarding the relative importance of metabolic pathways, species differences in toxic responses, rates of generation of reactive metabolites, and dose-dependent phenomena are highlighted. The first section briefly reviews information on the incidence and risk factors of kidney cancer in the general U.S. population. Epidemiological data on incidence of kidney cancer in male workers exposed occupationally to TCE are also summarized. This is contrasted with cancer bioassay data from laboratory animals, that highlights sex and species differences and, consequently, the difficulties in making risk assessments for humans based on animal data. The major section of the article considers proposed modes of action for TCE or its metabolites in kidney, including peroxisome proliferation, alpha(2u)-globulin nephropathy, genotoxicity, and acute and chronic toxicity mechanisms. The latter comprise oxidative stress, alterations in calcium ion homeostasis, mitochondrial dysfunction, protein alkylation, cellular repair processes, and alterations in gene expression and cell proliferation. Finally, the status of risk assessment for TCE based on the kidneys as a target organ and remaining questions and research needs are discussed.

Development of a physiologically based pharmacokinetic model of trichloroethylene and its metabolites for use in risk assessment

A physiologically based pharmacokinetic (PBPK) model was developed that provides a comprehensive description of the kinetics of trichloroethylene (TCE) and its metabolites, trichloroethanol (TCOH), trichloroacetic acid (TCA), and dichloroacetic acid (DCA), in the mouse, rat, and human for both oral and inhalation exposure. The model includes descriptions of the three principal target tissues for cancer identified in animal bioassays: liver, lung, and kidney. Cancer dose metrics provided in the model include the area under the concentration curve (AUC) for TCA and DCA in the plasma, the peak concentration and AUC for chloral in the tracheobronchial region of the lung, and the production of a thioacetylating intermediate from dichlorovinylcysteine in the kidney. Additional dose metrics provided for noncancer risk assessment include the peak concentrations and AUCs for TCE and TCOH in the blood, as well as the total metabolism of TCE divided by the body weight. Sensitivity and uncertainty analyses were performed on the model to evaluate its suitability for use in a pharmacokinetic risk assessment for TCE. Model predictions of TCE, TCA, DCA, and TCOH concentrations in rodents and humans are in good agreement with a variety of experimental data, suggesting that the model should provide a useful basis for evaluating cross-species differences in pharmacokinetics for these chemicals. In the case of the lung and kidney target tissues, however, only limited data are available for establishing cross-species pharmacokinetics. As a result, PBPK model calculations of target tissue dose for lung and kidney should be used with caution.

Role of maternal exposures and newborn genotypes on newborn chromosome aberration frequencies

Maternal exposures may induce chromosome damage and birth defects in the fetus. Polymorphic variation in genes coding for enzymes involved in metabolic activation and detoxification of environmental procarcinogens may account for some of the differences in chromosome aberration frequencies in newborns. In this study, 40 mothers completed questionnaires regarding exposures they received during their pregnancy. Umbilical cord blood samples were analyzed for chromosome aberrations. An average of 1020 metaphase cell equivalents (equal to 1020 G-banded cells) were examined from each newborn. In 26 of the newborns, genotyping analysis was performed for genes functioning in metabolic activation and detoxification (cytochrome P450 genes: CYP2D6 and CYP1A1, and phase II genes: NAT1, NAT2, GSTT1, GSTM1, GSTP1, and epoxide hydrolase). A significant association between the CYP1A1 MspI polymorphism and chromosome aberration frequencies was observed in the newborns (p=0.02), with heterozygotes showing higher aberration frequencies than the wild type homozygotes. Some large differences in chromosome aberration frequencies for other genotypes were also noted, but these were not statistically significant. Exposure to tobacco smoke in utero also appeared to increase translocation frequencies. The mean frequency of translocations per 100 cell equivalents from newborns of mothers who smoked during pregnancy was significantly higher than that of newborns whose mothers did not smoke (0.21 vs. 0.11, respectively, p=0.045).