Evaluation of fecal mutagenicity and colorectal cancer risk

Colorectal cancer is one of the most common internal malignancies in Western society. The cause of this disease appears to be multifactorial and involves genetic as well as environmental aspects. The human colon is continuously exposed to a complex mixture of compounds, which is either of direct dietary origin or the result of digestive, microbial and excretory processes. In order to establish the mutagenic burden of the colorectal mucosa, analysis of specific compounds in feces is usually preferred. Alternatively, the mutagenic potency of fecal extracts has been determined, but the interpretation of these more integrative measurements is hampered by methodological shortcomings. In this review, we focus on exposure of the large bowel to five different classes of fecal mutagens that have previously been related to colorectal cancer risk. These include heterocyclic aromatic amines (HCA) and polycyclic aromatic hydrocarbons (PAH), two exogenous factors that are predominantly ingested as pyrolysis products present in food and (partially) excreted in the feces. Additionally, we discuss N-nitroso-compounds, fecapentaenes and bile acids, all fecal constituents (mainly) of endogenous origin. The mutagenic and carcinogenic potency of the above mentioned compounds as well as their presence in feces, proposed mode of action and potential role in the initiation and promotion of human colorectal cancer are discussed. The combined results from in vitro and in vivo research unequivocally demonstrate that these classes of compounds comprise potent mutagens that induce many different forms of genetic damage and that particularly bile acids and fecapentaenes may also affect the carcinogenic process by epigenetic mechanisms. Large inter-individual differences in levels of exposures have been reported, including those in a range where considerable genetic damage can be expected based on evidence from animal studies. Particularly, however, exposure profiles of PAH and N-nitroso compounds (NOC) have to be more accurately established to come to a risk evaluation. Moreover, lack of human studies and inconsistency between epidemiological data make it impossible to describe colorectal cancer risk as a result of specific exposures in quantitative terms, or even to indicate the relative importance of the mutagens discussed. Particularly, the polymorphisms of genes involved in the metabolism of heterocyclic amines are important determinants of carcinogenic risk. However, the present knowledge of gene-environment interactions with regard to colorectal cancer risk is rather limited. We expect that the introduction of DNA chip technology in colorectal cancer epidemiology will offer new opportunities to identify combinations of exposures and genetic polymorphisms that relate to increased cancer risk. This knowledge will enable us to improve epidemiological study design and statistical power in future research

Modulation of nitrate-nitrite conversion in the oral cavity

Modulation of nitrate-nitrite conversion in the oral cavity. van Maanen JM, van Geel AA, Kleinjans JC. Department of Health Risk Analysis and Toxicology, University of Limburg, Maastricht, The Netherlands. The formation of nitrite from ingested nitrate can give rise to the induction of methemoglobinemia and endogenous nitrosation resulting in the formation of carcinogenic N-nitroso compounds. We investigated the possibility of modulation of the conversion of nitrate into nitrite in the oral cavity in order to seek ways of reducing the formation of the deleterious nitrite. We investigated the effectiveness of several mouthwash solutions with antibacterial constituents on the reduction of nitrate into nitrite in the oral cavity. In 15 studied subjects, the mean percentage of salivary nitrate reduced to nitrite after ingestion of 235 mg (3.8 mmol) nitrate was found to be 16.1 +/- 6.2%. The use of an antiseptic mouthwash with active antibacterial constituent chlorhexidine resulted in an almost complete decrease of the mean percentage of reduced nitrate, to 0.9 +/- 0.8%. Mouthwash solutions with antibacterial component triclosan or antimicrobial enzymes amyloglucosidase and glucose oxidase did not affect the reduction of nitrate into nitrite. A toothpaste with active components triclosan and zinc citrate with synergistic antiplaque activity was also without effect. Use of a pH-regulating chewing gum resulted in a rise in the pH in the oral cavity from 6.8 to 7.3. At 30 min after nitrate ingestion, this rise was accompanied by a significant increase in the salivary nitrite concentration, which might be explained by the pH being close to the optimal pH for nitrate reductase of 8. In conclusion, a limited number of possibilities of modulation of the conversion of nitrate into nitrite in the oral cavity are availabl

Formation of nitrosamines during consumption of nitrate- and amine-rich foods, and the influence of the use of mouthwashes.

Department of Health Risk Analysis and Toxicology, University of Maastricht, The Netherlands. We studied the formation of carcinogenic nitrosamines during consumption of food rich in nitrate and amines, and its possible inhibition by use of an antibacterial mouthwash. Twelve volunteers were fed a diet containing the high-nitrate vegetables lettuce or spinach during two periods of four consecutive days, in combination with fish products containing high levels of amines as nitrosatable precursors. During the two periods, the subjects used an antibacterial mouthwash containing chlorhexidine or a control mouthwash without antibacterial activity. Twenty-four-hour urine samples were collected after consumption of the meals, and saliva samples were collected 1 h after each meal. The nitrate and nitrite contents of the urine and saliva samples were determined by spectrophotometry (for nitrite) and HPLC (for nitrate). The concentrations of volatile nitrosamines in the urine samples were determined by gas chromatography-mass spectrometry. Significant increases in mean urinary nitrate levels (from 59 to 135 mg/24 h) and in mean salivary nitrate levels (from 10 to 56 microg/ml) and salivary nitrite levels (from 2 to 11 microg/ml) were observed during the consumption of food rich in nitrate and amines, as well as a significant increase in the mean urinary excretion of total examined volatile nitrosamines (from 2 to 7 nmol/24 h) and of N-nitrosodimethylamine (from 1.2 to 2.9 nmol/24 h). Use of the antibacterial mouthwash resulted in a decrease in mean salivary nitrite levels from 16 to 3 microg/ml and a decrease in mean urinary excretion of N-nitrosomorpholine (from 7.0 to 0.3 nmol/24 h). For the whole data set, significant correlations were observed between nitrate intake in food and urinary nitrate (p = 0.01; r2 = 0.07) and between urinary nitrate and urinary N-nitrosodimethylamine (p = 0.002; r2 = 0.11). In conclusion, consumption of a diet rich in nitrate and amines increases the risk of formation of carcinogenic nitrosamines. Use of an antibacterial mouthwash containing chlorhexidine can result in inhibition of nitrosamine formation

Pesticides and nitrate in groundwater and rainwater in the Province of Limburg in The Netherlands

The purpose of this study was to investigate the occurrence of high levels of pesticides in groundwater and rainwater in The Province of Limburg in The Netherlands. In groundwater samples in particular the presence of triazines - atrazine, simazine and propazine - was observed; besides these pesticides, dieldrin has also been observed. Atrazine and simazine were found to exceed the groundwater standard of 100 ng L-1. In the rainwater samples, the presence of 13 of 23 different analyzed pesticides was observed. A number of pesticides were found in high concentrations; e.g. atrazine (> 200 ng L-1). Two pesticides detected in rainwater (beta+gamma -HCH and atrazine) were found to exceed the groundwater standard. Seven pesticides in rainwater were found to exceed the target value and three pesticides the maximum tolerable risk value (DDT, heptachlor and heptachlorepoxide A), which are used as ecotoxicological standards in The Netherlands. Nitrate in 15 of 16 analyzed natural springs was found to exceed the guideline value for nitrate in drinking water of 50 mg L-1, up to levels of about 200 mg L-1. Nitrate concentrations in rainwater samples were observed up to 4.5 mg L-1. A risk analysis of exposure to high pesticide levels in groundwater or rainwater has been performed using the model HESP. For atrazine levels due to deposition of rainwater in two different locations, exceedance of the T.D.I. level of 0.5 mug kg(-1) day(-1) based on WHO criteria was observed for children using both an urban and a rural scenario and use of groundwater as drinking water