Effect of glutathione depletion on the irreversible association of acrylonitrile with tissue macromolecules after oral administration to rats

Acrylonitrile induction of rodent neoplasia: Potential mechanism of action and relevance to humans

Acrylonitrile, an industrial chemical, is a multisite carcinogen in rats and mice, producing tumors in four tissues with barrier function, that is, brain, forestomach, Zymbal’s gland, and Harderian gland. To assess mechanism(s) of action (MoA) for induction of neoplasia and to evaluate whether the findings in rodents are indicative of human hazard, data on the potential key effects produced by acrylonitrile in the four rodent target tissues of carcinogenicity were evaluated. A notable finding was depletion of glutathione in various organs, including two target tissues, the brain, and forestomach, suggesting that this effect could be a critical initiating event. An additional combination of oxidative DNA damage and cytotoxic effects of acrylonitrile and its metabolites, cyanide, and 2-cyanoethylene oxide, could initiate pro-inflammatory signaling and sustained cell and tissue injury, leading to compensatory cell proliferation and neoplastic development. The in vivo DNA-binding and genotoxicity of acrylonitrile has been studied in several target tissues with no compelling positive results. Thus, while some mutagenic effects were reported in acrylonitrile-exposed rodents, data to determine whether this mutagenicity stems from direct DNA reactivity of acrylonitrile are insufficient. Accordingly, the induction of tumors in rodents is consistent primarily with a non-genotoxic MoA, although a contribution from weak mutagenicity cannot be ruled out. Mechanistic data to support conclusions regarding human hazard from acrylonitrile exposure is weak. Comparison of metabolism of acrylonitrile between rodents and humans provide little support for human hazard. Three of the tissues affected in bioassays (forestomach, Zymbal’s gland, and Harderian gland) are present only in rodents, while the brain is anatomically different between rodents and humans, diminishing relevance of tumor induction in these tissues to human hazard. Extensive epidemiological data has not revealed causation of human cancer by acrylonitrile.

Tissue-Specific Metabolism of Benzene in Zymbal Gland and Other Solid Tumor Target Tissues in Rats

In vitro studies were carried out to investigate whether target organ susceptibility to benzene-induced solid tumor formation is governed by tissue-specific differences in metabolism. The ability of several target and nontarget tissues to deconjugate and conjugate polar metabolites, to metabolize benzene to phenolic metabolites, to carry out peroxidative biotransformations, and to trap tissue glutathione was evaluated. The Zymbal gland, the organ most sensitive to benzene-induced tumorigenicity, showed extensive phenyl- and aryl-sulfatase activity but no phenol sulfoconjugating activity. Similarly, oral cavity tissue, mammary gland, and bone marrow showed sulfatase activity but lacked sulfotransferase activity. Sulfatase-mediated hydrolysis such as that observed in the Zymbal gland may represent an important pathway by which polar metabolites are shunted from urinary or biliary excretion as their sulfates to delivery to target tissues as phenolic or potentially reactive metabolite(s). Zymbal gland, nasal and oral cavity, and mammary gland tissue homogenates (10,000 g supernatant) all possess oxidative capability to metabolize benzene to phenol, hydroquinone, and catechol. Nasal cavity homogenates produced two-to eightfold higher levels of phenol, hydroquinone, and catechol from benzene than did liver homogenates. Zymbal gland, bone marrow, and oral cavity homogenates, when incubated with hydroquinone and glutathione, produced high levels of 2-(S-glutathionyl)hydroquinone, indirectly indicating the production of 1,4-benzoquinone, a reactive intermediate implicated in benzene toxicity. Peroxidases have been proposed to mediate the oxidation of p-hydroquinone to 1,4-benzoquinone. The Zymbal gland, nasal and oral cavities, mammary gland, and bone marrow all were found to possess greater peroxidase activity than contrasting nontarget tissues did. The metabolic capabilities of target tissues, including the ability to hydrolyze sulfate conjugates to free phenolic compounds, to oxidize benzene to phenolic metabolites, to bioactivate hydroquinone to a reactive intermediate, and to carry out peroxidative reactions may offer possible explanations for the greater susceptibility of these sites to benzene-induced tumorigenicity. Transport of sulfate conjugates and their release via hydrolysis (e.g., through sulfatase action) (“sulfate shunting”) and subsequent oxidation (e.g., through peroxidase action) may represent a novel mechanistic pathway by which potentially reactive benzene metabolites can gain access to target sites and initiate critical genotoxic events.

Acrylonitrile-induced gastric toxicity in rats: the role of xanthine oxidase

BACKGROUND

Acrylonitrile (ACN) is an extensively produced aliphatic nitrile. The gastrointestinal tract is an important target organ for ACN toxicity. The objective of the present study was to investigate the role of xanthine oxidase (XO) in ACN-induced gastric toxicity in rats.

MATERIAL/METHODS

We assessed the effect of ACN on oxidative stress parameters as xanthine oxidase (XO) and total xanthine dehydrogenase (XD)/ XO activity, superoxide anion (O(2)(.-)) production, reduced glutathione (GSH) levels and lipid peroxidation in gastric tissues.

RESULTS

A single oral dose of ACN (25 mg/kg) caused a significant enhancement in XO activity. ACN also caused a significant depletion of GSH levels, enhanced O(2)(.-) production and increased lipid peroxidation in the time-course experiment. In the dose-response experiment, ACN accelerated the conversion of XD to XO, with a significant depletion of gastric GSH in a dose-related manner. A strong negative correlation existed between the levels of GSH and the percentage enhancement in XO activity (r =-0.997). (O(2)(.-)) production and malondialdehyde (MDA) formation were significantly elevated in a dose-related manner. Pretreatment with allopurinol (50 mg/kg) significantly protected against ACN-induced rise in XO activity, depletion of GSH, and elevated production of (O(2)(.-)). However, pretreatment with diethyl maleate (DEM; 100 mg/kg) significantly aggravated the ACN-induced GSH depletion and rise in XO activity. Furthermore, DEM significantly enhanced (O(2)(.-)) and MDA production.

CONCLUSIONS

The present study indicates that enhancement of XO activity could be implicated in ACN-induced gastric damage in rats.

Cancer dose–response assessment for acrylonitrile based upon rodent brain tumor incidence: Use of epidemiologic, mechanistic, and pharmacokinetic support for nonlinearity

A cancer dose-response assessment was conducted for acrylonitrile (AN) using updated information on mechanism of action, epidemiology, toxicity, and pharmacokinetics. Although more than 10 chronic bioassays indicate that AN produces multiple tumors in rats and mice, a number of large, well-conducted epidemiology studies provide no evidence of a causal association between AN exposure and cancer mortality of any type. The epidemiological data include early industry exposures that are far higher than occur today and that approach or exceed levels found to be tumorigenic in animals. Despite the absence of positive findings in the epidemiology data, a dose-response assessment was conducted for AN based on brain tumors in rats. Mechanistic studies implicate the involvement of oxidative stress in rat brain due to a metabolite (2-cyanoethylene oxide or CEO, cyanide), but do not conclusively rule out a potential role for the direct genotoxicity of CEO. A PBPK model was used to predict internal doses (peak CEO in brain) for 12 data sets, which were pooled together to provide a consistent characterization of the dose-response relationship for brain tumor incidence in the rat. The internal dose corresponding to a 5% increase in extra risk (ED 05=0.017 mg/L brain) and its lower confidence limit (LED 05=0.014 mg/L brain) was used as the point of departure. The weight-of-evidence supports the use of a nonlinear extrapolation for the cancer dose-response assessment. A quantitative comparison of the epidemiology exposure-response data (lung and brain cancer mortality) to the rat brain tumor data in terms of internal dose adds to the confidence in the nonlinear extrapolation. Uncertainty factors of 200 and 220 (for the oral and inhalation routes, respectively) were applied to the LED 05 to account for interspecies variation, intraspecies variation, and the severity of the response measure. Accordingly, oral doses below 0.009 mg/kg-day and air concentrations below 0.1mg/m(3) are not expected to pose an appreciable risk to human populations exposed to AN.

A pharmacokinetic modeling of inorganic arsenic: a short-term oral exposure model for humans

This study presents a pharmacokinetic modeling of inorganic arsenic disposition in human body for short-term oral exposures. Effort on the development of the model is directed toward the prediction of the kinetic behavior of inorganic arsenic and its metabolites in the body. The current model considers the 4 circulating species; AsIII, AsV, and two metabolites such as monomethylarsenic (MMA) and dimethylarsenic (DMA) in the blood and tissue groups. While it is difficult to estimate some parameters used in the models at this time, the current model assumptions and predictions seem to be consistent with the experimental observations found in the literature. Hence, the current model, when more fully developed, is expected to provide insight into the behavior of inorganic arsenic and its methylated metabolites within the body, and may help increase the understanding of risk assessment issues associated with inorganic arsenic in drinking water.

Co-oxidation of acrylonitrile by soybean lipoxygenase and partially purified human lung lipoxygenase

1. Human lung lipoxygenase (HLLO) was partially purified by concanavalin-A (Con-A) affinity chromatography that provided an easy and rapid one-step procedure for the removal (> or = 96%) of haemoglobin from cytosol. 2. HLLO exhibited dioxygenase activity towards arachidonic acid (AA) and linoleic acid (LA). The dioxygenase activity towards LA varied approximately 12-fold (48-591 nmol/min/mg protein) among different human lung samples examined. 3. Reverse-phase HPLC analysis of AA metabolites indicated the predominance of 15-lipoxygenase in human lung cytosol. 4. HLLO exhibited co-oxidase activity towards benzidine (BZD) and several other model compounds. The co-oxidase activity towards BZD was significantly inhibited by several lipoxygenase inhibitors. 5. HLLO and soybean lipoxygenase (SLO), used as a model enzyme, metabolized acrylonitrile (ACN) to 2-cyanoethylene oxide (CEO) and ultimately to cyanide. 6. HLLO was a approximately 6-fold better catalyst than SLO in converting ACN to cyanide. The generation of cyanide by HLLO was dependent on the concentration of enzyme and the reaction was inhibited by the lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA) and the anti-oxidant butylated hydroxytoluene (BHT). 7. Under optimal assay conditions, the covalent binding of HLLO-generated reactive intermediate(s) from [14C]ACN to protein and DNA (nmol equivalent bound/15 min/mg HLLO/mg bovine serum albumin or calf thymus DNA) was observed at approximately 1.20+/-0.13 and 2.20+/-0.50 respectively. Both protein and DNA binding were inhibited by NDGA, butylated hydroxyanisole (BHA) and BHT.

Evaluation of possible genotoxic mechanisms for acrylonitrile tumorigenicity

Acrylonitrile (ACN) exposure is associated with tumors in rat brain, Zymbal gland, and mammary gland. Adducts affecting base pairing were formed in isolated DNA exposed in vitro to the ACN metabolite cyanoethylene oxide (CNEO). DNA from liver, which is not a cancer target organ in ACN-exposed rats, contained low levels of 7-(2-oxoethyl)guanine, and adduct believed not to interfere with base pairing. No adducts have been detected in brain DNA from ACN-exposed rats, suggesting that brain tumors may have arisen by mechanisms other than ACN-DNA reactivity. Genotoxicity assays of ACN have indicated no particular carcinogenic mechanism. Positive reverse mutagenesis in Salmonella typhimurium HisG46 base substitution tester strains by ACN is attributable to CNEO. Other in vitro genotoxicity test assays of ACN have yielded mixed results, without consistent effect of metabolic activation. Some positive genotoxicity data for ACN appear to result from artifacts or from non-DNA-reactive mechanisms. In vivo micronucleus, chromosome aberration, and autoradiographic unscheduled DNA synthesis assays were negative for ACN. The comparative genotoxicity of vinyl chloride and ACN indicates that despite other similarities, they cause rodent tumors by different mechanisms. Also, they absence of ACN-DNA adduct formation in the rat brain suggests the operation of epigenetic mechanisms.

Modulation of acrylonitrile-induced embryotoxicity in vitro by glutathione depletion

The effects of glutathione (GSH) depletion on the embryotoxicity of acrylonitrile were assessed in vitro using the rat whole-embryo culture system. Day 10 rat embryos were cultured in rat serum medium for 6 h in the presence of 250 microM L-buthionine-S,R-sulfoximine (BSO), a specific inhibitor of GSH synthesis, to deplete GSH in both embryo and visceral yolk sac. Following pretreatment, conceptuses were cultured for an additional 21 h in the presence of 152, 228, or 304 microM acrylonitrile. At the end of the culture period, conceptuses were assessed for survival, growth and development, malformations, and the protein and glutathione content of embryos and yolk sacs were assayed. Acrylonitrile alone produced concentration-related and statistically significant decreases in yolk sac diameter, crown-rump length, head length and number of somite pairs, as well as in embryonic and yolk sac proteins. The chemical also caused dysmorphogenesis of the brain and of the caudal extremity, and a concentration-related and statistically significant increase in GSH content in the yolk sac. Pretreatment with BSO significantly enhanced the embryotoxic effects of acrylonitrile. The conceptuses displayed further decreases in functional yolk sac circulation, yolk sac diameter, crown-rump and head length, when compared to either acrylonitrile or BSO alone. The incidence of caudal malformations and the severity of brain malformations produced by acrylonitrile were also increased. Marked decreases in embryonic and yolk sac GSH contents were observed after exposure to BSO alone or in combination with acrylonitrile.(ABSTRACT TRUNCATED AT 250 WORDS)