Effect of glutathione modulation on molecular interaction of [14C]-chloroacetonitrile with maternal and fetal DNA in mice

Single and combined effects of selected haloacetonitriles in a human-derived hepatoma line

Haloacetonitriles (HANs) are nitrogenous disinfection byproducts (N-DBPs) detected in drinking water that have high toxicity and are a high risk to human health. The cytotoxicity and genotoxicity as well as the oxidative stress of five HANs, namely chloroacetonitrile (CAN), dichloroacetonitrile (DCAN), trichloroacetonitrile (TCAN), bromoacetonitrile (BAN), and dibromoacetonitrile (DBAN) on a hepatoma cell line (HepG2) were determined by single, binary or ternary exposure. The median effective concentrations, based on cell viability, ranged from 0.8360 mg/L for BAN to 256.9 mg/L for DCAN, with a cytotoxicity order of BAN > DBAN > CAN > TCAN > DCAN. The lowest observed effective concentrations regarding DNA damage were 0.01 mg/L for CAN and DCAN, 0.1 mg/L for DBAN and TCAN, and 1 mg/L for BAN. The DNA damage induced by CAN, DCAN and TCAN was repaired to about 80% in 30 min, and when induced by BAN and DBAN, it was repaired completely in 60 min. The intracellular reactive oxygen species (ROS) levels were significantly increased by the five HANs, and bromo-acetonitrile produced a stronger oxidative stress than chloro-acetonitrile. Co-exposure of DCAN, TCAN and DBAN significantly inhibited cell viability, induced DNA damage and facilitated ROS generation in HepG2 cells. However, the interactive effects were inconsistent for the different endpoints, which seemed to be antagonism for cell viability but synergy for ROS generation.

Energy of the Lowest Unoccupied Molecular Orbital, Thiol Reactivity, and Toxicity of Three Monobrominated Water Disinfection Byproducts

Disinfection of drinking water protects public health against waterborne pathogens. However, during disinfection, toxic disinfection byproducts (DBPs) are formed. Exposure to DBPs was associated with increased risk of bladder cancer in humans. DBPs are generated at concentrations below their carcinogenic potencies; it is unclear how exposure leads to adverse health outcomes. We used computational estimates of the energy of the lowest unoccupied molecular orbital (ELUMO) to predict thiol reactivity and additive toxicity among soft electrophile DBPs. Bromoacetic acid (BAA) was identified as non-thiol-reactive, which was supported by in chemico and in vitro data. Bromoacetonitrile (BAN) and bromoacetamide (BAM) were thiol-reactive. Genotoxicity induced by these compounds was reduced by increasing the thiol pool with N-acetyl L-cysteine (NAC), while NAC had little effect on BAA. BAN and BAM shared depletion of glutathione (GSH) or cellular thiols as a molecular initiating event (MIE), whereas BAA induces toxicity through another pathway. Binary mixtures of BAM and BAN expressed a potentiating effect in genotoxicity. We found that soft electrophile DBPs could be an important predictor of common mechanism groups that demonstrated additive toxicity. In silico estimates of ELUMO could be used to identify the most relevant DBPs that are the forcing factors of the toxicity of finished drinking waters.

Comparison of DNA damage in human-derived hepatoma line (HepG2) exposed to the fifteen drinking water disinfection byproducts using the single cell gel electrophoresis assay

Disinfection of drinking water reduces pathogenic infection, but generates disinfection by-products (DBPs) in drinking water. In this study, the effect of fifteen DBPs on DNA damage in human-derived hepatoma line (HepG2) was investigated by the single cell gel electrophoresis (SCGE) assay. These fifteen DBPs are: four trihalomethanes (THMs), six haloacetic acides (HAAs), three haloacetonitriles (HANs), 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), and chloral hydrate (CH). Based on the minimal effective concentration (MEC) at which DBPs induced significant increase in olive tail moment (OTM), the rank order of DNA-damaging potency is: bromodichloromethane (BDCM)>dibromochloromethane (DBCM)>tribromomethane (TBM)>trichloromethane (TCM) of the four THMs; iodoacetic acid (IA)>bromoacetic acid (BA)>dibromoacetic acid (DBA)>dichloracetic acid (DCA)>trichloroacetic acid (TCA) of the five HAAs; dibromoacetonitrile (DBN)approximately dichloroacetonitrile (DCN)>trichloroacetonitrile (TCN) of the three HANs. The DNA damaging potency of MX and CH is similar to TCA and DCA, respectively. IA is the most genotoxic DBP in the fifteen DBPs, followed by BA. Chloroacetic acid (CA) is not genotoxic in this assay. Our findings indicated that HepG2/SCGE is a sensitive tool to evaluate the genotoxicity of DBPs and iodinated DBPs are more genotoxic than brominated DBPs, but chlorinated DBPs are less genotoxic than brominated DBPs.

Haloacetonitriles: metabolism and toxicity

The haloacetonitriles (HANs) exist in drinking water exclusively as byproducts of disinfection. HANs are found in drinking water more often, and in higher concentrations, when surface water is treated by chloramination. Human exposure occurs through consumption of finished drinking water; oral and dermal contact also occurs, and results from showering, swimming and other activities. HANs are reactive and are toxic to gastrointestinal tissues following oral administration. Such toxicity is characterized by GSH depletion, increased lipid peroxidation, and covalent binding of HAN-associated radioactivity to gut tissues. The presence of GSH in cells is an important protective mechanism against HAN toxicity; depletion of cellular GSH results in increased toxicity. Some studies have demonstrated an apparently synergistic effect between ROS and HAN administration, that may help explain effects observed in GI tissues. ROS are produced in gut tissues, and in vitro evidence indicates that ROS may contribute to the degradation and formation of reactive intermediates from HANs. The rationale for ROS involvement may involve HAN-induced depletion of GSH and the role of GSH in scavenging ROS. In addition to effects on GI tissues, studies show that HAN-derived radiolabel is found covalently bound to proteins and DNA in several organs and tissues. The addition of antioxidants to biologic systems protects against HAN-induced DNA damage. The protection offered by antioxidants supports the role of oxidative stress and the potential for a threshold in han-induced toxicity. However, additional data are needed to substantiate evidence for such a threshold. HANs are readily absorbed from the GI tract and are extensively metabolized. Elimination occurs primarily in urine, as unconjugated one-carbon metabolites. Evidence supports the involvement of mixed function oxidases, the cytochrome P450 enzyme family and GST, in HAN metabolism. Metabolism represents either a detoxification or bioactivation process, depending on the particular HAN and the enzyme involved. HANs can inhibit CYP2E1-mediated metabolism, an effect which may be dependent on a covalent interaction with the enzyme. In addition, HAN compounds inhibit GST-mediated conjugation, but this effect is reversible upon dialysis, indicating that the interaction does not represent covalent binding. No subchronic studies of HAN toxicity are available in the literature. However, studies show that HANs produce developmental toxicity in experimental animals. The nature of developmental toxicity is affected by the type of administration vehicle, which renders interpretation of results more difficult. Skin tumors have been found following dermal application of HANs, but oral studies for carcinogenicity are negative. Pulmonary adenomas were increased following oral administration of HANs, but the A/J strain of mice employed has a characteristically high background rate of such tumors. HANs interact with DNA to produce unscheduled DNA repair, SCE and reverse mutations in Salmonella. HANs did not induce micronuclei or cause alterations in sperm head morphology in mice, but did induce micronuclei in newts. Thus, there is concern for the potential carcinogenicity of HANs. It would be valuable to delineate any relationship between the apparent threshold for micronuclei formation in newts and the potential mechanism of toxicity involving HAN-induced oxidative stress. Dose-response studies in rodents may provide useful information on toxicity mechanisms and dose selection for longer term toxicity studies. Additional studies are warranted before drawing firm conclusions on the hazards of HAN exposure. Moreover, additional studies on HAN-DNA and HAN-protein interaction mechanisms, are needed. Such studies can better characterize the role of metabolism in toxicity of individual HANs, and delineate the role of oxidative stress, both of which enhance the capacity to predict risk. Most needed, now, are new subchronic (and chronic) toxicity studies; the results of such well-planned, controlled, conducted, interpreted and published investigations would be valuable in establishing margins of safety for HANs in human health risk assessment.

Chloroacetonitrile induces intrauterine growth restriction and musculoskeletal toxicity in fetal mouse

Chloroacetonitrile (CAN) is a by-product of chlorination of drinking water. Epidemiological studies indicate that it might present a hazard to human health. The present study was designed to investigate the potential adverse effects of intrauterine exposure to CAN on fetal body weight and development of the musculoskeletal system in mice. At gestation day 6, pregnant mice were given CAN (12.5, 25, or 50 mg/kg/day) till gestation day 18. Uteri were then examined and live fetuses were collected, weighed, and evaluated for any malformations. High doses of CAN (50 mg/kg) significantly elevated fetal anomalies and reduced fetal viability. Chloroacetonitrile at a dose of 25 mg/kg did not affect fetal viability and significantly reduced fetal body weight. Subsequent experimentation was performed using this dose level. Histological examination of fetal axial skeleton indicated that CAN resulted in delayed appearance of endochondral ossification centers, widening of the vertebrae, and destruction of the calcified zone. In addition, the skeletal muscle fibers were markedly distorted, were small in size, and were widely separated by connective tissue. Both connective tissue perimysium and endomysium were less cellular compared with control sections. The histological findings were further confirmed by assessing the morphometric changes. Ratios of calcified cartilage to non-calcified cartilage areas in both control and CAN-exposed groups were determined. Also, skeletal muscle fiber diameter of CAN-exposed fetuses was significantly decreased compared with control group. In conclusion, intrauterine exposure to low levels of CAN decreases fetal body weight and induces malformations in the musculoskeletal system in mice.

Fetal origin of adverse pregnancy outcome: the water disinfectant by-product chloroacetonitrile induces oxidative stress and apoptosis in mouse fetal brain

Epidemiological studies indicate a relationship between water disinfectant by-products (DBP) and adverse pregnancy outcomes (APO) including neural tube defects. These studies suggest that fetal brain may be vulnerable to DBP during early stages of development. Therefore, we examined several molecular markers commonly known to indicate chemical-induced neurotoxicity during fetal brain development following prenatal exposure to the DBP; chloroacetonitrile (CAN). Pregnant mice, at gestation day 6 (GD6), were treated with a daily oral dose of CAN (25 mg/kg). At GD12, two groups of animals were treated with an i.v. tracer dose of [2-14C]-CAN. These animals were sacrificed at 1 and 24 h after treatment and processed for quantitative in situ micro-whole-body autoradiography. The remaining groups of animals continued to receive CAN. At GD18, control and treated animals were weighed, anesthetized, and fetuses were obtained and their brains were removed for biochemical and immunohistochemical analyses. Whole-body autoradiography studies indicate a significant uptake and retention of [2-14C]-CAN/metabolites (M) in fetal brain (cerebral cortex, hippocampus, cerebellum) at 1 and 24 h. There was a 20% reduction in body weight and a 22% reduction in brain weight of fetuses exposed to CAN compared to controls. A significant increase in oxidative stress markers was observed in various fetal brain regions in animals exposed to CAN compared to controls. This was indicated by a 3- to 4-fold decrease in the ratio of the reduced to oxidized form of glutathione (GSH/GSSG), increased lipid peroxidation (1.3-fold), and increased 8-hydroxy-2-deoxyguanosine levels (1.4-fold). Cupric silver staining indicated a significant increase in the number of degenerating neurons in cortical regions in exposed animals. In animals exposed to CAN there was increase in nuclear DNA fragmentation (TUNEL staining) detected in the cerebral cortex and cerebellum (2-fold increase in apoptotic indices). Caspase-3 activity in cerebral cortex and cerebellum of treated animals were also increased (1.7- and 1.5-fold, respectively). In conclusion, this study indicates that CAN/M crossed the placenta and accumulated in fetal brain tissues where it caused oxidative stress and neuronal apoptosis. These events could predispose the fetus to altered brain development leading to APO as well as behavioral and learning and memory deficits.

Neurological impairment in fetal mouse brain by drinking water disinfectant byproducts

Developmental exposure to environmental chemicals may have detrimental effects on embryonic brains that could play a major role in the etio-pathology of fetal and adult neurological diseases. The exact mechanism by which prenatal exposures to environmental agents, such as drinking water disinfectant byproducts (DBP), cause neurological impairment in fetus is not known. Our objective is to examine the impact of prenatal exposure to DBP on fetal brain development. Pregnant CD-1 mice, at the sixth day of gestation (GD-6), received a daily (GD-6-GD-18) oral dose of chloroacetonitrile (CAN, 25 ppm), a member of DBP. To assess fetal brain uptake of CAN, several animals were injected with a tracer dose of 2-[(14)C]-CAN (333 microCi/kg, i.v.), at GD-12 and processed for quantitative in situ micro whole-body autoradiography (QIMWBA) at 1 and 24 h after treatment. The remaining animals continued receiving CAN until GD-18 when fetal brains were processed for biochemical determination of oxidative stress (OS) or prepared for histological examinations. The results indicate a rapid placental transfer and fetal brain uptake of 2-[(14)C]-CAN/metabolites in cortical areas and hippocampus. In treated animals 3-fold decrease in glutathione (GSH), 1.3-fold increase in lipid peroxidation and 1.4-fold increase in DNA oxidation were detected as compared to control. DeOlmos cupric silver staining of fetal brains indicated significant increase in cortical neurodegeneration in treated animals. Immunohistochemical labeling (TUNEL) of apoptotic nuclei in the cortices and choroid plexuses were also increased in treated animals as compared to control. In conclusion, CAN crosses the placental and fetal blood-brain barriers and induces OS that triggered apoptotic neurodegenration in fetal brain. Future studies will examine the molecular mechanisms of these events and its impact on neural development of offspring.

Chloroacetonitrile (CAN) induces glutathione depletion and 8-hydroxylation of guanine bases in rat gastric mucosa

Chloroacetonitrile (CAN) is detected in drinking-water supplies as a by-product of the chlorination process. Gastroesophageal tissues are potential target sites of acute and chronic toxicity by haloacetonitriles (HAN). To examine the mechanism of CAN toxicity, we studied its effect on glutathione (GSH) homeostasis and its impact on oxidative DNA damage in gastric mucosal cells of rats. Following a single oral dose (38 or 76 mg/Kg) of CAN, animals were sacrificed at various times (0-24 h), and mucosa from pyloric stomach were collected. The effects of CAN treatment on gastric GSH contents and the integrity of genomic gastric DNA were assessed. Oxidative damage to gastric DNA was evaluated by measuring the levels of 8-Hydroxydeoxyguanosine (8-OHdG) in hydrolyzed DNA by HPLC-EC. The results indicate that CAN induced a significant, dose- and time-dependent, decrease in GSH levels in pyloric stomach mucosa at 2 and 4 hours after treatment (56 and 39% of control, respectively). DNA damage was observed electrophoretically at 6 and 12 hours following CAN administration. CAN (38 mg/Kg) induced significant elevation in levels of 8-OHdG in gastric DNA. Maximum levels of 8-OHdG in gastric DNA were observed at 6 hours after CAN treatment [9.59+/-0.60 (8-OHdG/10(5)dG) 146% of control]. When a high dose of CAN (76 mg/Kg) was used, a peak level of 8-OHdG [11.59+/-1.30 (8-OHdG/10(5)dG) 177% of control] was observed at earlier times (2 h) following treatment. When CAN was incubated with gastric mucosal cells, a concentration-dependent cyanide liberation and significant decrease in cellular ATP levels were detected. These data indicate that a mechanism for CAN-induced toxicity may be partially mediated by depletion of glutathione, release of cyanide, interruption of the energy metabolism, and induction of oxidative stress that leads to oxidative damage to gastric DNA.

Genotoxic activity of five haloacetonitriles: comparative investigations in the single cell gel electrophoresis (comet) assay and the ames-fluctuation test

Halogenated acetonitriles (HANs) are known to be water disinfectant by-products. Their mutagenicity and carcinogenicity have been shown in different test systems in vivo and in vitro. They also have clastogenic properties. In this study, the ability of HAN to induce single-strand breaks on the DNA of HeLa S3 cells was investigated using the single-cell gel electrophoresis (SCGE) assay, which could be a good tool with which to evaluate the genotoxicity of chlorinated water. The results were compared to those obtained in the Ames fluctuation test using the Salmonella typhimurium TA 100 strain without activation. With the Ames fluctuation test, a mutagenic effect was observed for chloroacetonitrile (MCAN), dichloroacetonitrile (DCAN), and trichloroacetonitrile (TCAN). No mutagenic effect was found with bromoacetonitrile (MBAN) or dibromoacetonitrile (DBAN). In the SCGE assay, all five HANs induced DNA damage in HeLa S3 cells, increasing the mean tail moment significantly. For each compound, a dose-effect relation was observed. This study shows that the SCGE assay has greater sensitivity for assessing the genotoxicity of HAN than does the Ames-fluctuation test. Brominated acetonitriles were more genotoxic than chlorinated acetonitriles in the SCGE assay, and the genotoxicity increased with the number of halogenated atoms of the compound. This behavior had already been found with other genotoxicity tests.

Chloroacetonitrile-induced toxicity and oxidative stress in rat gastric epithelial cells

Chloroacetonitrile (CAN), is a disinfectant by-product of chlorination of drinking water. Epidemiological studies indicate that exposure to CAN via drinking water might present a potential hazard to human health. The objective of the present work was to investigate the cytotoxic effects as well as the oxidative stress induced by CAN in cultured rat gastric epithelial cells (GECs). GECs were exposed in vitro to different concentrations of CAN (5-40 microm) for 60 min. Also, GECs were incubated with CAN (10 microm) for different time intervals extending to 180 min. Cytotoxicity was determined by assessing cell viability and lactate dehydrogenase (LDH) release, glutathione (GSH) level and lipid peroxidation as indicated by malondialdehyde (MDA) production. Exposure of GECs CAN (10 microm) for 60 min caused a 50% decrease in cell viability and induced an eightfold increase of LDH leakage. In the same experiment, CAN caused a decrease in cellular GSH content to approximately 50% and significantly enhanced MDA accumulation (approx. sevenfold). These toxic responses to CAN were dependent on both concentration and duration of exposure to CAN. There was a good correlation between LDH release and GSH depletion (r =0.96, P<0.05). Treatment of GECs with 5 m mN -acetyl- l -cysteine (NAC) prior to exposure to CAN afforded some degree of protection as indicated by a significant decrease in the LDH leakage (32% of total leakage) and lipid peroxidation (54%) as compared to CAN alone-treated cells. Also, pretreatment of GECs with vitamin C (1 m m) or vitamin E (10 microm) significantly inhibited LDH leakage (20 and 36% of total leakage, respectively). Preincubation with 1 m m desferroxiamine (DFO), a ferric iron chelator, or 10 microm phenanthroline (PHE), a ferrous iron chelator, diminished CAN-induced LDH leakage by 16 and 21% of total leakage, respectively and MDA production by 40 and 44%, respectively. In conclusion, our results suggest that CAN has a potential cytotoxic effect in rat GECs; and thiol group-donors, antioxidants and iron chelators can play a critical role against CAN-induced cellular damage.

Effect of glutathione modulation of the distribution and transplacental uptake of 2-[14C]-chloroacetonitrile (CAN) quantitative whole-body autoradiographic study in pregnant mice

Chloroacetonitrile (CAN), a drinking water disinfectant by-product, has mutagenic and carcinogenic properties. CAN is known to deplete glutathione (GSH), and previous studies reported an enhanced molecular interaction of CAN after GSH depletion in the uterine and fetal tissues of mice. The present report may help to understand the potential mechanisms involved in such molecular interactions by examining the disposition, transplacental uptake and covalent interaction of the chemical in normal and GSH depleted pregnant mice (at 13th day of gestation). Both normal and GSH depleted (by administration of Diethylmaleate (DEM), 0.6 mL/kg, i.p.) pregnant mice were given an equitoxic i.v. dose of 2-[14C]-CAN(333 microCi/kg equivalent to 77 mg/kg). Animals were processed for whole-body autoradiography (WBA) at 1, 8 and 24 hr after treatment. Tissue distribution of radioactivity in the autoradiographs was quantitated using computer aided image analysis. With few exceptions, a rapid high uptake (at 1 hr) of radioactivity was observed in all major maternal (liver, lung, urinary bladder, gastrointestinal mucosa, cerebellum, uterine luminal fluid) and fetal (liver, brain) organs of both normal and GSH depleted mice. This pattern of distribution was observed, with lesser intensity, at 8 hr following treatment. At a later time period (24 hr), there was a significant higher retention and covalent interaction of radioactivity in GSH depleted mouse tissues especially in the liver as compared to normal mouse. This study suggests that 2-[14C]-CAN and/or its metabolites are capable of crossing the placental barrier. The observed higher uptake and retention of the radioactivity in the maternal liver, kidney, cerebellum, nasal turbinates and fetal liver may pose toxicity of the chemical to these organs. The increased covalent interaction of radioactivty in GSH depleted mice liver may indicate the potential utilization of GSH pathway by this organ in the detoxication of CAN derived metabolites and thus exerting hepatotoxicity.