Molecular biomarkers of oxidative stress associated with bromate carcinogenicity.

Potassium bromate (KBrO3) is a chemical oxidizing agent found in drinking water as a disinfection byproduct of surface water ozonation. Chronic exposures to KBrO3 cause renal cell tumors in rats, hamsters and mice and thyroid and testicular mesothelial tumors in rats. Experimental evidence indicates that bromate mediates toxicological effects via the induction of oxidative stress. To investigate the contribution of oxidative stress in KBrO3-induced cancer, male F344 rats were administered KBrO3 in their drinking water at multiple concentrations for 2-100 weeks. Gene expression analyses were performed on kidney, thyroid and mesothelial cell RNA. Families of mRNA transcripts differentially expressed with respect to bromate treatment included multiple cancer, cell death, ion transport and oxidative stress genes. Multiple glutathione metabolism genes were up-regulated in kidney following carcinogenic (400 mg/L) but not non-carcinogenic (20 mg/L) bromate exposures. 8-Oxodeoxyguanosine glycosylase (Ogg1) mRNA was up-regulated in response to bromate treatment in kidney but not thyroid. A dramatic decrease in global gene expression changes was observed following 1mg/L compared to 20 mg/L bromate exposures. In a separate study oxygen-18 (18O) labeled KBrO3 was administered to male rats by oral gavage and tissues were analyzed for 18O deposition. Tissue enrichment of 18O was observed at 5 and 24 h post-KBr18O3 exposure with the highest enrichment occurring in the liver followed by the kidney, thyroid and testes. The kidney dose response observed was biphasic showing similar statistical increases in 18O deposition between 0.25 and 50 mg/L (equivalent dose) KBr18O3 followed by a much greater increase above 50 mg/L. These results suggest that carcinogenic doses of potassium bromate require attainment of a threshold at which oxidation of tissues occurs and that gene expression profiles may be predictive of these physiological changes in renal homeostasis.

Tribromomethane exposure and dietary folate deficiency in the formation of aberrant crypt foci in the colons of F344/N rats.

Folate and folic acid are forms of the B vitamin that are involved in the synthesis, repair, and functioning of DNA and are required for the production and maintenance of cells. Low levels of folate have been associated with several forms of cancer, including colon cancer. Aberrant crypt foci (ACF), identified as putative precursor lesions in the development of colon cancer, have been induced by the drinking water disinfection by-product, tribromomethane (TBM). To investigate whether ACF induced by TBM could be promoted by a diet devoid of dietary folate, male F344/N rats were exposed to 500 mg/l of TBM in drinking water and fed either a normal or no folate diet (NFD) for 26 weeks. At the conclusion of the study, colons were excised and examined for ACF. Rats exposed to TBM and fed a NFD, evident by significantly reduced serum folate concentrations and elevated serum homocysteine levels, had significant increases of ACF when compared to rats exposed to TBM and fed a normal diet. This study highlights the important role that diet, especially folate intake, represents in protecting the colon against TBM-induced ACF.

The effects of a high animal fat diet on the induction of aberrant crypt foci in the colons of male F344/N rats exposed to trihalomethanes in the drinking water.

Aberrant crypt foci (ACF), identified as putative precursor lesions in the development of colon cancer, were induced by brominated trihalomethanes (THMs) administered in the drinking water of rats. To investigate whether ACF induced by THMs could be promoted by a diet high in saturated animal fat, male F344/N rats were exposed to 0.5, 0.7, 0.9 or 1.1 g/l of trichloromethane (TCM), bromodichloromethane (BDCM), dibromochloromethane (DBCM) and tribromomethane (TBM), respectively, in drinking water. All animals were fed a Purina 5001 diet with half receiving the normal 4.5% fat feed and half receiving feed supplemented with 19% animal fat. After 26 weeks of treatment, colons were excised and examined for ACF. No difference in ACF was noted between animals fed a normal or high fat diet and exposed to TCM, BDCM or DBCM. However, animals exposed to TBM and fed a high fat diet showed a significant and near two-fold increase in ACF when compared to TBM exposed animals fed a normal diet.

Vehicle and mode of administration effects on the induction of aberrant crypt foci in the colons of male F344/N rats exposed to bromodichloromethane.

Bromodichloromethane (BDCM) and tribromomethane given by corn oil gavage were previously found to induce neoplasia in the large intestine of rats. Our chronic bioassay of BDCM administered in drinking water failed to produce colon neoplasia in male F344/N rats. We recently reported that BDCM induces aberrant crypt foci (ACF), putative precursor lesions in the development of colon cancer, when included in the drinking water of male rats. To investigate whether ACF induced by BDCM could be promoted by corn oil (CO), male F344/N rats were exposed to 0.7 g BDCM/L in drinking water or 50 mg BDCM/kg body weight by oral gavage in CO. Animals exposed to drinking water, CO, or 15 mg/kg azoxymethane (AOM) (ip) constituted the negative, vehicle, and positive controls. After 26 wk, colons were examined for ACF. A significant decrease in water consumption was observed in both the positive controls and BDCM-treated animals; however, no difference was noted in final body weight. The administration of CO to AOM-exposed animals produced a significant increase in total ACF when compared to AOM alone. BDCM produced a significant increase in ACF when compared to control, but no difference was noticed between BDCM exposure by oral CO gavage and control. Additionally, no difference was noted between BDCM exposure by drinking water and by oral CO gavage. This study demonstrates that the formation of ACF is independent of the route of BDCM exposure (drinking water vs. oral corn oil gavage), with both routes producing similar ACF values of 1.33 +/- 0.49 and 1.5 +/- 0.51 ACF/colon.

Carcinogenicity of bromodichloromethane administered in drinking water to Male F344/N Rats and B6C3F1 mice.

A life-time exposure study was conducted to assess the carcinogenicity of bromodichloromethane (BDCM) administered in the drinking water to male F344/N rats and B6C3F(1) mice. In mouse, the calculated mean daily BDCM concentrations (measured concentrations corrected for on-cage loss of chemical) were 0.06, 0.28 and 0.49 g/l. Time-weighted water consumption of 135, 97, and 89 ml/kg/day resulted in mean daily doses of 8.1, 27.2, and 43.4 mg BDCM/kg/day. No changes in feed consumption, final body weight, or survival were observed. Kidney weights were significantly depressed at 27.2 and 43.4 mg BDCM/kg/day. There was no increase in neoplasia in the liver, kidney, spleen, testis, bladder, sections along the alimentary tract, excised lesions, or at any other organ site. In rat, the corrected mean daily BDCM concentrations were 0.06, 0.33, and 0.62 g/l. Time-weighted water consumption of 65, 63, and 59 ml/kg/day yielded 3.9, 20.6 and 36.3 mg BDCM/kg/day. No alterations in feed consumption, body weight gain, and survival were seen. Kidney weight was significantly depressed in the 36.3-mg/kg/day treatment group. There was a significantly enhanced prevalence and multiplicity of hepatocellular adenomas at 3.9 mg BDCM/kg/day (15.5% and 0.16/animal vs. 2.2% and 0.02/animal for the control). Hepatocellular carcinomas increased from 2.2% and 0.02/animal for the control and 3.9 mg BDCM/kg/day to 8.3% and 0.10/animal at 20.6 mg BDCM/kg/day. The combined neoplasms were enhanced at 3.9 and 20.6 mg BDCM/kg/day. Liver neoplasia was depressed to the control value at 36.3 mg BDCM/kg. The prevalence of basophilic and clear cell, but not eosinophilic cells, altered foci of cells declined with increasing dose. BDCM did not increase cancer in the large bowel, renal tubules, or in any of the other tissues examined. Renal tubular hyperplasia was observed at 36.3 mg BDCM/kg (15.8% vs. 8.7% for the control group). Under the conditions of the study, BDCM in the drinking water was not carcinogenic in the male B6C3F(1) mouse, but was carcinogenic in the male F344/N rat based on an increased hepatocellular neoplasia.