Hans-Günter Neumann

The major metabolite of aflatoxin B1 in the rat is a glutathione conjugate

[14C]aflatoxin B1 (AFB1) was injected i.p. into female Wistar rats. Half of the dose was eliminated into the bile mostly as polar non-extractable metabolites. Among these a glutathione conjugate was the main component. The same conjugate was formed when rat liver postmitochondrial supernatant was incubated with AFB1 and [3H]glutathione. The conjugate was purified by ion exchange chromatography, gel-filtration and thin layer chromatography (TLC). It was tentatively identified as 2,3-dihydro-2-(S-glutathionyl)-3-hydroxy aflatoxin B1 (AFB1-GSH-conjugate). This structure was derived mainly from amino acid analysis, ultraviolet spectra and the enzymatic requirements for its formation in in vitro experiments. In the rat this detoxification product of the potentially ultimate reactive AFB1-epoxide constitutes about 10% of the administered dose and thus underlines the quantitative importance of this activating pathway.

POLYCYCLIC NITROARENES (NITRO-PAHS) AS BIOMARKERS OF EXPOSURE TO DIESEL EXHAUST

Diesel exhaust contains numerous genotoxic carcinogens. It is essentially unknown to which extent this source contributes to the total load of these chemicals in humans. One possible approach to the problem is to find suitable biomarkers. To this end five polycyclic mononitroarenes (nitro-PAH) were selected and methods developed to determine the sulfinic acid-type hemoglobin adducts they form in vivo. The nitro-PAHs are: 1-nitropyrene, 2-nitrofluorene, 3-nitrofluoranthene, 9-nitrophenanthrene, and 6-nitrochrysene. Hydrolysis of the hemoglobin adducts yields the respective arylamines which were analyzed by gas chromatography/mass spectrometry. The detection limit was 0.01-0.08 pmol/g Hb. Blood samples were analyzed from 29 bus garage workers, occupationally exposed to diesel exhaust, and from 20 urban hospital workers and 14 rural council workers as controls. Hb adducts above the detection limit were found in most blood samples. The most abundant cleavage products were 1-aminopyrene and 2-aminofluorene with levels ranging from 0.01 to 0.68 pmol/g Hb. However, there was no significant difference between the groups for 1-nitropyrene and 2-nitrofluorene supporting the conclusion that both are widespread environmental contaminants resulting in significant background exposures. A significant difference on a group from individuals from urban and rural areas was found only if all five adducts were added, this may indicate an additional exposure from traffic. The new specific nitro-PAH Hb adducts are proposed to be used as biomarkers to trace the sources and to identify above-background exposures.

Early resistance to cell death and to onset of the mitochondrial permeability transition during hepatocarcinogenesis with 2-acetylaminofluorene

A hallmark of tumorigenesis is resistance to apoptosis. To explore whether resistance to cell death precedes tumor formation, we have studied the short-term effects of the hepatocarcinogen 2-acetylaminofluorene (AAF) on liver mitochondria, on hepatocytes, and on the response to bacterial endotoxin lipopolysaccharide (LPS) in albino Wistar rats. We show that after as early as two weeks of AAF feeding liver mitochondria developed an increased resistance to opening of the permeability transition pore (PTP), an inner membrane channel that is involved in various forms of cell death. Consistent with a mitochondrial adaptive response in vivo, (i) AAF feeding increased the expression of BCL-2 in mitochondria, and (ii) hepatocytes isolated from AAF-fed rats became resistant to PTP-dependent depolarization, cytochrome c release, and cell death, which were instead observed in hepatocytes from rats fed a control diet. AAF-fed rats were fully protected from the hepatotoxic effects of the injection of 20–30 μg of LPS plus 700 mg of d-galactosamine (d-GalN) × kg–1 of body weight, a treatment that in control rats readily caused a large increase of terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling-positive cells in liver cryosections and release of alanine and aspartate aminotransferase into the bloodstream. Treatment with LPS and d-GalN triggered cleavage of BID, a BCL-2 family member, in the livers of both control- and AAF-fed animals, whereas caspase 3 was cleaved only in control-fed animals, indicating that the mitochondrial proapoptotic pathway had been selectively suppressed during AAF feeding. Phenotypic reversion was observed after stopping the carcinogenic diet. These results underscore a key role of mitochondria in apoptosis and demonstrate that regulation of the mitochondrial PTP is altered early during AAF carcinogenesis, which matches, and possibly causes, the increased resistance of hepatocytes to death stimuli in vivo. Both events precede tumor formation, suggesting that suppression of apoptosis may contribute to the selection of a resistant phenotype, eventually increasing the probability of cell progression to the transformed state.

Biomonitoring of aromatic amines II: hemoglobin binding of some monocyclic aromatic amines

Covalent binding of 13 monocyclic aromatic amines to hemoglobin was studied in female Wistar rats and hemoglobin binding indices were determined. The hemoglobin adducts were hydrolyzed under alkaline conditions. In all cases the parent amine could be identified by gas chromatography and with one exception represented the only cleavage product. The binding index varied considerably and was highest withp-chloroaniline (569) and lowest with 2,4,5-trimethylaniline (0.7). Five compounds were also studied in female B6C3F1 mice. Hemoglobin binding was lower than in rats, but to varying degrees. Hemoglobin binding correlated remarkably well with the maximum methemoglobin level achieved with the six examples studied. The results support the notion that the reaction of nitrosoarenes, as metabolites of arylamines, with hemoglobin represents a general pathway in vivo. The analysis of such hemoglobin adducts is recommended as a dosimeter in biological monitoring of humans in order to control exposure. It is too early, however, to assess the carcinogenic risk from hemoglobin binding data with these compounds.

The role of DNA damage in chemical carcinogenesis of aromatic amines

SummaryMany findings support the notion that the generation of DNA adducts by aromatic amines is causally related to carcinogenesis. Adducts have been identified in most cases and representative examples are reviewed. However, extent and persistence of DNA adducts (DNA dose) does not correlate satisfactorily with the tumor response of different tissues. Distribution of DNA damage, repair, indirect and secoridary DNA damage are discussed as possible explanations for the observed noncorrelations. In addition, however, it is proposed to pay attention to specific mechanisms such as receptor mediated cellular effects which are not related to the generation of electrophiles. The effects of trans-4-aminostilbene and 2-amino-fluorene derivatives on rat liver are compared. It is concluded that trans-4-acetylamino-stilbene is a strong liver tumor initiator but an incomplete liver carcinogen lacking tumor promoting protperties, and that 2-acetylaminofluorene is a complete liver carcinogen with initiating and promoting properties.

Biomonitoring of aniline and nitrobenzene

Covalent binding to hemoglobin was studied to further substantiate the proposal that it may be used for biomonitoring N-substituted aryl compounds. (14C)-Labeled acetanilide and nitrobenzene were orally administered to female Wistar rats and binding indices [Binding(mmol/mol Hb)/Dose(mmol/kg)] determined; these were 12±1 and 73±10, respectively. After mild acidic or alkaline hydrolysis, 90% of the bound material was released and identified as aniline by radio thin layer chromatography. This supports the hypothesis that nitroso aryl derivatives, common intermediates in the metabolism of N-substituted aryl compounds, react with SH-groups of hemoglobin to yield sulfinic acid amides. Aniline was furthermore identified and quantified by capillary gas chromatography, using hemoglobin from animals treated with unlabeled aniline and nitrobenzene. Binding indices in this case were 30±3 and 85±19, respectively. With this method human blood samples may also be analysed.Although nitrobenzene is known to produce less methemoglobin than aniline, hemoglobin binding is higher. This indicates that hemoglobin binding may be a better index of body burden than methemoglobin levels in biomonitoring N-substituted aryl compounds.

BIOMarkers for occupational diesel exhaust exposure monitoring (BIOMODEM)--a study in underground mining.

Methods for the assessment of exposures to diesel exhaust were evaluated, including various biomarkers of internal exposure and early biological effects. The impact of possible biomarkers of susceptibility was also explored. Underground workers (drivers of diesel-powered excavators) at an oil shale mine in Estonia were compared with surface workers. Personal exposures to particle-associated 1-nitropyrene (NP) were some eight times higher underground than on the surface. Underground miners were also occupationally exposed to benzene and polycyclic aromatic hydrocarbons, as indicated by excretion of urinary metabolites of benzene and pyrene. In addition, increased O(6)-alkylguanine DNA adducts were detected in the white blood cells of underground workers, suggesting higher exposure to nitroso-compounds. However, no differences between underground and surface workers were observed in the levels of other bulky DNA adducts determined by 32P-postlabelling, or in DNA damage. The study indicated that smoking, diet and residential indoor air pollution are important non-occupational factors to consider when interpreting biomonitoring results.

A METABOLITE OF CARCINOGENIC 2-ACETYLAMINOFLUORENE, 2-NITROSOFLUORENE, INDUCES REDOX CYCLING IN MITOCHONDRIA

The present study was designed to confirm the recent proposal that 2-nitrosofluorene (2-NOF) as well as N-hydroxy-2-aminofluorene (N-OH-AF) induce a redox-cycle in rat liver mitochondria as part of the chronic toxic effects of the carcinogen 2-acetylaminofluorene (2-AAF). The formation of O2.- was demonstrated in submitochondrial particles by the formation of adrenochrome with NADH and succinate as respiratory substrates. 2-NOF was as effective as paraquat, a known redox-cycler, the lowest effective concentration being 0.4 nmol 2-NOF/mg protein. Experiments with isolated mitochondria showed that 2-NOF, in contrast to N-OH-AF, induces cyanide-resistant O2 consumption only in the presence of respiratory substrates, indicating that the reduction, but not the reoxidation, depends on a continuous flow of electrons through the respiratory chain of the mitochondrial membrane. Lipid peroxidation was estimated by the formation of thiobarbituric-acid-reactive substances. In comparison to the well-known prooxidant tert-butylhydroperoxide, 2-NOF was not significantly active. The results support the notion that 2-NOF induces oxidative stress by mitochondrial redox-cycling in vivo. Effects other than lipid peroxidation seem to be important for the chronic toxicity of 2-AAF.

Changes in the classification of carcinogenic chemicals in the work area

Abstract Carcinogenic chemicals in the work area are currently classified into three categories in section III of the German List of MAK and BAT Values (list of values on maximum workplace concentrations and biological tolerance for occupational exposures). This classification is based on qualitative criteria and reflects essentially the weight of evidence available for judging the carcinogenic potential of the chemicals. It is proposed that these categories – IIIA1, IIIA2, IIIB – be retained as Categories 1, 2, and 3, to correspond with European Union regulations. On the basis of our advancing knowledge of reaction mechanisms and the potency of carcinogens, these three categories are supplemented with two additional categories. The essential feature of substances classified in the new categories is that exposure to these chemicals does not contribute significantly to risk of cancer to man, provided that an appropriate exposure limit (MAK value) is observed. Chemicals known to act typically by nongenotoxic mechanisms and for which information is available that allows evaluation of the effects of low-dose exposures, are classified in Category 4. Genotoxic chemicals for which low carcinogenic potency can be expected on the basis of dose-response relationships and toxicokinetics, and for which risk at low doses can be assessed are classified in Category 5.The basis for a better differentiation of carcinogens is discussed, the new categories are defined, and possible criteria for classification are described. Examples for Category 4 (1,4-dioxane) and Category 5 (styrene) are presented.

Biomonitoring of aromatic amines III: Hemoglobin binding of benzidine and some benzidine congeners

Covalent binding of benzidine and some congeners to hemoglobin was studied in female Wistar rats after oral administration. Hemoglobin adducts were hydrolyzed under alkaline conditions, and the arylamines extracted and analysed by HPLC with electrochemical detector. With benzidine, three cleavage products were observed, the major component being monoacetylbenzidine. This indicates that 4-nitroso-4′-N-acetylaminobiphenyl is the major reactive metabolite in erythrocytes. In addition benzidine and 4-aminobiphenyl were identified. The latter indicates a hitherto unknown metabolic pathway of benzidine. With 3,3′-dichlorobenzidine-dihydrochloride, 3,3′-dimethoxybenzidine and 3,3′-dimethylbenzidine two cleavage products were observed, the parent diamines being present in excess to or in amounts comparable to the monoacetyl derivative. With 3,3′,5,5′-tetramethylbenzidine a hemoglobin adduct could not be found. When the azo dye direct red 28 was administered to the animals, the three cleavage products typical for benzidine were found, indicating that benzidine became bioavailable after reductive cleavage of the azo compound. In this case the fraction of 4-aminobiphenyl was greater than after benzidine. It is proposed to use the analysis of hemoglobin adducts in human blood to control the exposure of individuals to these carcinogenic chemicals in the course of biochemical effect monitoring.