Biochemical effects of three carcinogenic chlorinated methanes in rat liver

Lack of in vivo mutagenicity of 1,2-dichloropropane and dichloromethane in the livers of gpt delta rats administered singly or in combination

1,2-Dichloropropane (1,2-DCP) and dichloromethane (DCM) are possible causative agents associated with the development of cholangiocarcinoma in employees working in printing plant in Osaka, Japan. However, few reports have demonstrated an association between these agents and cholangiocarcinoma in rodent carcinogenicity studies. Moreover, the combined effects of these compounds have not been fully elucidated. In the present study, we evaluated the in vivo mutagenicity of 1,2-DCP and DCM, alone or combined, in the livers of gpt delta rats. Six-week-old male F344 gpt delta rats were treated with 1,2-DCP, DCM or 1,2-DCP + DCM by oral administration for 4 weeks at the dose (200 mg kg^-1 body weight 1,2-DCP and 500 mg kg^-1 body weight DCM) used in the carcinogenesis study performed by the National Toxicology Program. In vivo mutagenicity was analyzed by gpt mutation/Spi^- assays in the livers of rats. In addition, gene and protein expression of CYP2E1 and GSTT1, the major enzymes responsible for the genotoxic effects of 1,2-DCP and DCM, were analyzed by quantitative polymerase chain reaction and western blotting. Gpt and Spi^- mutation frequencies were not increased by 1,2-DCP and/or DCM in any group. Additionally, there were no significant changes in the gene and protein expression of CYP2E1 and GSTT1 in any group. These results indicated that 1,2-DCP, DCM and 1,2-DCP + DCM had no significant impact on mutagenicity in the livers of gpt delta rats under our experimental conditions. Copyright © 2016 John Wiley & Sons, Ltd.

Assessment of the genotoxicity of 1,2-dichloropropane and dichloromethane after individual and co-exposure by inhalation in mice

OBJECTIVE

Occurrence of cholangiocarcinoma was recently reported at a high incidence rate among the employees working for an offset printing company in Osaka, Japan. 1,2-Dichloropropane (1,2-DCP) and dichloromethane (DCM) are suspected to be the causes of the cancer, as they had been used as ink cleaners in large amounts. However, it is not clear whether these chlorinated organic solvents played a role in the occurrence of cholangiocarcinoma or why the incidence rate is so high among the workers in this industry. To provide possible evidence for this severe occupational problem, we investigated the genotoxic effects of 1,2-DCP and DCM.

METHODS

Male B6C3F1 and gpt Delta C57BL/6J mice were exposed by inhalation to the individual solvents or both solvents at multiple concentrations including the levels that were possibly present in the workplaces. The genotoxicity was analyzed by Pig-a gene mutation and micronuclei assays in peripheral blood and gpt mutation and comet assays in the livers of mice after repeated inhalation of 1,2-DCP or/and DCM.

RESULTS

The Pig-a mutant frequencies and micronuclei incidences were not significantly increased by exposure of either 1,2-DCP or/and DCM at any concentration, suggesting there was no genotoxic potential in bone marrow for both solvents. In the liver, DNA damage, as measured by the comet assay, was dose dependently increased by 1,2-DCP but not by DCM. The gpt mutant frequency was 2.6-fold that of the controls in the co-exposure group.

CONCLUSIONS

These results indicate that 1,2-DCP showed stronger genotoxicity in the liver and that the genotoxic effects were greatly enhanced by simultaneous exposure to DCM.

Zonal location of compensatory hepatocyte proliferation following chemically induced hepatotoxicity in rats and humans

Hepatocyte proliferation stimulated by partial hepatectomy occurs first in periportal cells, with midlobular and then perivenous cell division occurring later. We have previously shown that this pattern of compensatory cell proliferation also occurs following the hepatotoxicity of N-nitrosodimethylamine. We examined the generality of this pattern in livers of rats given a minimally toxic dose of an hepatotoxin and in liver biopsy samples from patients who had taken overdoses of acetaminophen. Proliferating hepatocytes were detected immunohistochemically (5'-bromodeoxyuridine or Ki-67 antigens). The perivenous necrogens N-nitrosodiethylamine, carbon tetrachloride (CCl4), bromobenzene, and acetaminophen all induced periportal hepatocyte proliferation. With CCl4, bromobenzene, and acetaminophen, the initial appearance of proliferating periportal hepatocytes was followed 12-24 hr later by division in the midlobular region, with a few cells dividing adjacent to the perivenous region of necrosis. The periportal necrogen allyl alcohol also induced periportal cell division. In the human studies, perivenous necrosis was accompanied by periportal and midlobular hepatocyte proliferation. These results suggest that regardless of its lobular location chemically induced hepatotoxicity stimulates cell proliferation that begins in the periportal zone and then moves in an orchestrated response into the midlobular and perivenous zones.

Dose-response relationship for rat liver DNA damage caused by 1,2-dimethylhydrazine

An experimental approach was taken to the question of dose-response curves for chemical carcinogenesis, using DNA damage as a biomarker. Female rats were give 13 different doses of 1,2-dimethylhydrazine (from 1.4 to 135,000 micrograms/kg) and the subsequent hepatic DNA damage was determined by the alkaline elution technique. DMH doses below 450 micrograms/kg did not significantly damage DNA; all DMH doses of 1000 micrograms/kg or higher damaged rat hepatic DNA (P < 0.05). In this study the x values (dose) ranged over five orders of magnitude and the y values (DNA damage) ranged 30-fold. Ten different regression models (linear, quadratic, cubic, power, and six nonlinear transition models) were compared in their ability to fit the experimental data. With respect to log transformed dose, the six nonlinear transition equations fit the data considerably better than the four power type of equations. A sigmoid model fit to the log transformed dose of 1,2-dimethylhydrazine had an r2 of 0.9979, a degree of freedom adjusted r2 of 0.9969, a F-statistic of 1,457, and a fit standard error of 0.50. With respect to untransformed dose, only three equations (sigmoid, cascade and gaussian cumulative) could creditably fit the DMH data. The experimental results are interpreted with respect to hormesis, use of log transformed dose, sigmoid dose-response models, thresholds of biological response and cancer risk assessment.

Dose-response relationship for rat liver DNA damage caused by 49 rodent carcinogens

An experimental approach was taken to the question of dose-response curves for chemical carcinogenesis. DNA damage in female rat liver was chosen as the experimental parameter because all chemicals found to damage hepatic DNA were rodent carcinogens. The lowest dose causing DNA damage was determined for the 12 active chemicals (1,2-dibromoethane, 1,2-dibromo-3-chloropropane, 1,2-dichloroethane, 1,4-dioxane, methylene chloride, auramine O, Michler's ketone, selenium sulfide, 1,3-dichloropropene, 1,2-dimethylhydrazine, N-nitroso-piperidine and butylated hydroxytoluene). The resulting dose-response curves for rat hepatic DNA damage were plotted versus log of the molar dose (all activity was in five orders of magnitude) and versus percent of chemicals' oral rat LD50 (most of the activity was in only two orders of magnitude). Dose-response studies of the active chemicals were analyzed by regression methods. With the exception of butylated hydroxytoluene, the dose-response curves fit a linear model well (r2 = 0.886) and a quadratic model even better (r2 = 0.947). Based on experimental data from 11 DNA-damaging carcinogens (a dose range of 6 orders of magnitude), an equation and graph of the dose-response relationship of an 'average DNA-damaging carcinogen' is presented over the x-axis dose range of eight orders of magnitude.

Complementarity of genotoxic and nongenotoxic predictors of rodent carcinogenicity

Twenty-one chemicals carcinogenic in rodent bioassays were selected for study. The chemicals were administered by gavage in two dose levels to female Sprague-Dawley rats. The effects of these 21 chemicals on four biochemical assays [hepatic DNA damage by alkaline elution (DD), hepatic ornithine decarboxylase activity (ODC), serum alanine aminotransferase activity (ALT), and hepatic cytochrome P-450 content (P450)] were determined. Available data from seven cancer predictors published by others [the Ames test (AMES), mutation in Salmonella typhimurium TA 1537 (TA 1537), structural alerts (SA), mutation in mouse lymphoma cells (MOLY), chromosomal aberrations in Chinese hamster ovary cells (ABS), sister chromatid exchange in hamster ovary cells (SCE), and the ke test (ke)] were also compiled for these 21 chemical carcinogens plus 28 carcinogens and 62 noncarcinogens already published by our laboratory. From the resulting 111 (chemicals) by 11 (individual cancer predictors) data matrix, the five operational characteristics (sensitivity, specificity, positive predictivity, negative predictivity, and concordance) of each of the 11 individual cancer predictors (four biochemical parameters of this study and seven cancer predictors of others) are presented. Two examples of complementarity or synergy of composite cancer predictors were found. To obtain maximum concordance it was necessary to combine both genotoxic and nongenotoxic cancer predictors. The composite cancer predictor (DD or [ODC and P450] or [ODC and ALT]) had higher concordance than did any of the four individual cancer predictors from which it was constructed. Similarly, the composite cancer predictor (TA 1537 or DD or [ODC and P450] or [ODC and ALT]) had higher concordance than any of its five individual constituent cancer predictors. Complementarity or synergy has been demonstrated both 1) among genotoxic cancer predictors (DD and TA 1537) and 2) between nongenotoxic (ODC, P450, and ALT) and genotoxic cancer predictors (TA 1537 and DD).

Predicting rodent carcinogenicity of Ames test false positives by in vivo biochemical parameters

28 chemicals known to be mutagenic in the Ames test but not carcinogenic in rodent bioassays were selected for study. The chemicals were administered by gavage in 2 dose levels to female Sprague-Dawley rats. The effects of these 28 chemicals on 4 biochemical assays (hepatic DNA damage by alkaline elution (DD), hepatic ornithine decarboxylase activity (ODC), serum alanine aminotransferase activity (ALT), and hepatic cytochrome P-450 content (P450)) were determined. The scientific approach taken was to either experimentally find individual cancer predictors of high specificity or to mathematically create composite predictors of high specificity. Composite predictive parameters are defined as follows: CP = [ODC and P450], CT = [ALT and ODC], and TS = [DD or CP or CT]. The specificity (percent of rodent noncarcinogens which test negative) of DD, ODC, ALT, P450, CP, CT and TS was 100%, 46%, 89%, 86%, 93%, 93% and 86%, respectively. For these 28 mutagenic noncarcinogens, the specificity of structural alerts (SA) 13%, mutation in mouse lymphoma cells (MOLY) 0%, chromosomal aberrations in Chinese hamster ovary cells (ABS) 13%, and sister-chromatid exchange in Chinese hamster ovary cells (SCE) 0% were much lower. The ke test, an experimental measure of electron attachment, had a specificity of 33%. DD was the only DNA related parameter to predict well the noncarcinogenic rodent bioassay result of Ames false-positive chemicals. 5 nongenotoxic parameters (ALT, P450, CP, CT and [CP or CT]) predicted the rodent bioassay result well. Depending on the prevalence of chemicals carcinogenic to humans, the problem of Ames test false positives for predicting human cancer may be either small or large.

Predicting rodent carcinogenicity of halogenated hydrocarbons by in vivo biochemical parameters

Forty halogenated hydrocarbons of known rodent carcinogenicity (24 carcinogens, 16 noncarcinogens), including many promoters of carcinogenesis, nongenotoxic carcinogens, and hepatocarcinogens, were selected for study. The chemicals were administered by gavage in two dose levels to female Sprague-Dawley rats. The effects of these 40 chemicals on four biochemical assays [hepatic DNA damage by alkaline elution (DD), hepatic ornithine decarboxylase activity (ODC), serum alanine aminotransferase activity (ALT), and hepatic cytochrome P-450 content (P450)] were determined. Composite predictive parameters are defined as follows: CP = [ODC and P450], CT = [ALT and ODC], and TS = [DD or CP or CT]. The operational characteristics of TS for predicting rodent cancer were sensitivity 58%, specificity 81%, positive predictivity 82%, negative predictivity 57%, and concordance 68%. The concordance for the Ames test (45%) and structural alerts (SA; 46%) was much lower. TS also outperformed the Ames test and SA in producing fewer false positives (the specificity of TS was 81% vs. only 63% for the Ames test and 57% for SA). For predicting the carcinogenicity of the most difficult halogenated hydrocarbons (Ames and SA negative chemicals), TS was capable of successfully predicting the carcinogenicity of 8 (carbon tetrachloride, chloroform, alpha-hexachlorocyclohexane, kepone, mirex, monuron, p,p'-DDE, and 2,4,6-trichlorophenol) out of 16 of these non-DNA-reactive halogenated hydrocarbon carcinogens. All 8 of these halogenated hydrocarbons were positive in either CP or CT. This evidence shows that nongenotoxic carcinogenesis is best predicted by nongenotoxic parameters such as CP or CT (components of the predictor TS).

Predictive assay for rodent carcinogenicity using in vivo biochemical parameters: operational characteristics and complementarity

111 chemicals of known rodent carcinogenicity (49 carcinogens, 62 noncarcinogens), including many promoters of carcinogenesis, nongenotoxic carcinogens, hepatocarcinogens, and halogenated hydrocarbons, were selected for study. The chemicals were administered by gavage in two dose levels to female Sprague-Dawley rats. The effects of these 111 chemicals on 4 biochemical assays (hepatic DNA damage by alkaline elution (DD), hepatic ornithine decarboxylase activity (ODC), serum alanine aminotransferase activity (ALT), and hepatic cytochrome P-450 content (P450)) were determined. Composite parameters are defined as follows: CP = [ODC and P450), CT = [ALT and ODC), and TS = [DD or CP or CT]. The operational characteristics of TS for predicting rodent cancer were sensitivity 55%, specificity 87%, positive predictivity 77%, negative predictivity 71%, and concordance 73%. For these chemicals, the 73% concordance of this study was superior to the concordance obtained from published data from other laboratories on the Ames test (53%), structural alerts (SA) (46%), chromosome aberrations in Chinese hamster ovary cells (ABS) (48%), cell mutation in mouse lymphoma 15178Y cells (MOLY) (52%), and sister-chromatid exchange in Chinese hamster ovary cells (SCE) (60%). The 4 in vivo biochemical assays were complementary to each other. The composite parameter TS also shows complementarity to all 5 other predictors of rodent cancer examined in this paper. For example, the Ames test alone has a concordance of only 53%. In combination with TS, the concordance is increased to 62% (Ames or TS) or to 63% (Ames and TS). For the 67 chemicals with data available for SA, the concordance for predicting rodent carcinogenicity was 47% (for SA alone), 54% (for SA or TS), and 66% (for SA and TS). These biochemical assays will be useful: (1) to predict rodent carcinogenicity per se, (2) to 'confirm' the results of short-term mutagenicity tests by the high specificity mode of the biochemical assays (the specificity and positive predictivity are both 100%), and (3) to be a component of future complementary batteries of tests for predicting rodent carcinogenicity.

Biochemical studies of six nitrogen-containing heterocycles in rat tissues

Female rats were dosed orally with one-fifth the LD50 of either 1-nitrosopiperidine (a carcinogen), cyclohexylamine, piperidine, 4-carboxy-1-nitrosopiperidine, 4-cyclohexyl-1-nitrosopiperidine or 2,6-dimethyl-1-nitrosopiperidine at 21 and 4 hr before they were killed. The five noncarcinogenic compounds had no effects on any experimental variables examined [hepatic DNA damage, ornithine decarboxylase (ODC) activity, serum alanine aminotransferase (SGPT) activity, cytochrome P-450 and glutathione content]. After administration of 40 mg/kg of 1-nitrosopiperidine, marked hepatic DNA damage and a 3- to 7-fold increase in the activity of hepatic ODC were observed. 1-Nitrosopiperidine (120 mg/kg, 3/5 LD50) caused DNA damage in rat liver and esophagus but not in leukocytes. This higher dose of 1-nitrosopiperidine also increased hepatic ornithine decarboxylase activity by 9-fold. Thus, this hepatic biochemical assay system correctly identified the one carcinogen and the five noncarcinogens in this series of six nitrogen-containing heterocycles.

Biochemical studies of promoters of carcinogenesis in rat liver

Adult female rats were orally dosed with 1/5 to 3/5 the published LD50 of either promoters or putative promoters of carcinogenesis [hexachlorobenzene (HCB), alpha-hexachlorocyclohexane (alpha-HCH), kepone and toxaphene] or noncarcinogens [coumaphos, EDTA, caprolactam, 8-hydroxyquinoline, titanium (IV) oxide, sodium diethyldithiocarbamate (DEDTC), and sucrose] at 21 and 4 h before sacrifice. The promoters selected in this study were all of the halogenated hydrocarbon class. At doses of 1/5 to 3/5 the LD50, all four promoters or putative promoters induced rat hepatic ODC activity. The seven noncarcinogens produced several biochemical effects at doses of 1/5 the LD50: increased serum alanine aminotransferase activity (SGPT) (caprolactam and DEDTC), decreased hepatic cytochrome P-450 content (DEDTC), and increased hepatic ODC activity (8-hydroxyquinoline and DEDTC). None of the seven noncarcinogens caused hepatic DNA damage or coordinate induction of hepatic ODC and cytochrome P-450. The results support the interpretation that several of these biochemical parameters are useful in distinguishing potential tumor promoters and noncarcinogens.