Metabolism of dietary genotoxins by the human colonic microflora; the fecapentaenes and heterocyclic amines

Links between Diet, Intestinal Anaerobes, Microbial Metabolites and Health

A dense microbial community resides in the human colon, with considerable inter-individual variability in composition, although some species are relatively dominant and widespread in healthy individuals. In disease conditions, there is often a reduction in microbial diversity and perturbations in the composition of the microbiota. Dietary complex carbohydrates that reach the large intestine are important modulators of the composition of the microbiota and their primary metabolic outputs. Specialist gut bacteria may also transform plant phenolics to form a spectrum of products possessing antioxidant and anti-inflammatory activities. Consumption of diets high in animal protein and fat may lead to the formation of potentially deleterious microbial products, including nitroso compounds, hydrogen sulphide, and trimethylamine. Gut anaerobes also form a range of secondary metabolites, including polyketides that may possess antimicrobial activity and thus contribute to microbe-microbe interactions within the colon. The overall metabolic outputs of colonic microbes are derived from an intricate network of microbial metabolic pathways and interactions; however, much still needs to be learnt about the subtleties of these complex networks. In this review we consider the multi-faceted relationships between inter-individual microbiota variation, diet, and health.

Mechanistic Evidence for Red Meat and Processed Meat Intake and Cancer Risk: A Follow-up on the International Agency for Research on Cancer Evaluation of 2015

The Working Group of the International Agency for Research on Cancer classified the consumption of processed meat as carcinogenic to humans (Group 1), and classified red meat as probably carcinogenic to humans (Group 2A); consumption of both meat types is associated with an increased risk of colorectal cancer. These classifications are based on a compilation of epidemiology data and mechanistic evidence from animal and human studies. The curing of meats with nitrite can produce carcinogenic N-nitroso compounds (NOCs), and the smoking of meat produces polycyclic aromatic hydrocarbons (PAHs). The high-temperature cooking of meat also produces carcinogenic heterocyclic aromatic amines (HAAs). The ingestion of heme from meat can catalyze the formation of NOCs and lipid peroxidation products (LPOs) in the digestive tract. Many of these chemicals form DNA adducts, some of which can induce mutations and initiate carcinogenesis. Another recent hypothesis is that N-glycolylneuraminic acid, a non-human sialic acid sugar present in red meat, becomes incorporated in the cell membrane, triggering the immune response with associated inflammation and reactive oxygen species, which can contribute to DNA damage, tumor promotion, and cancer. The mechanisms by which these chemicals in meat induce DNA damage, and the impact of dietary and host factors that influence the biological potency of these chemicals are highlighted in this updated report.

Mechanisms and therapeutic effectiveness of lactobacilli

The gut microbiome is not a silent ecosystem but exerts several physiological and immunological functions. For many decades, lactobacilli have been used as an effective therapy for treatment of several pathological conditions displaying an overall positive safety profile. This review summarises the mechanisms and clinical evidence supporting therapeutic efficacy of lactobacilli. We searched Pubmed/Medline using the keyword ‘Lactobacillus’. Selected papers from 1950 to 2015 were chosen on the basis of their content. Relevant clinical and experimental articles using lactobacilli as therapeutic agents have been included. Applications of lactobacilli include kidney support for renal insufficiency, pancreas health, management of metabolic imbalance, and cancer treatment and prevention. In vitro and in vivo investigations have shown that prolonged lactobacilli administration induces qualitative and quantitative modifications in the human gastrointestinal microbial ecosystem with encouraging perspectives in counteracting pathology-associated physiological and immunological changes. Few studies have highlighted the risk of translocation with subsequent sepsis and bacteraemia following probiotic administration but there is still a lack of investigations on the dose effect of these compounds. Great care is thus required in the choice of the proper Lactobacillus species, their genetic stability and the translocation risk, mainly related to inflammatory disease-induced gut mucosa enhanced permeability. Finally, we need to determine the adequate amount of bacteria to be delivered in order to achieve the best clinical efficacy decreasing the risk of side effects.

Role of intestinal microflora in xenobiotic-induced toxicity

In addition to its role in digestion of food in the gastrointestinal tract, the intestinal microflora is also capable of biotransforming numerous drugs. Likewise, the intestinal microflora may significantly modulate xenobiotic-induced toxicity by either activating or inactivating xenobiotics via metabolism. To date, most investigations of xenobiotic metabolism have focused not only on metabolism in host tissues, but the modulation of the pharmacological activity of drugs by the intestinal microflora. Despite its importance, the presumed role of intestinal microflora metabolism in xenobiotic-induced toxicity has been understudied. Therefore, it is appropriate to briefly review our current situation, and state which research in xenobiotic metabolism by intestinal microflora, particularly in the field of toxicology, is needed.

Potential role of probiotics on colorectal cancer prevention

Background

Colorectal cancer represents the most common malignancy of the gastrointestinal tract. Owing to differences in dietary habits and lifestyle, this neoplasm is more common in industrialized countries than in developing ones. Evidence from a wide range of sources supports the assumption that the link between diet and colorectal cancer may be due to an imbalance of the intestinal microflora.

Discussion

Probiotic bacteria are live microorganisms that, when administered in adequate amounts, confer a healthy benefit on the host, and they have been investigated for their protective anti-tumor effects. In vivo and molecular studies have displayed encouraging findings that support a role of probiotics in colorectal cancer prevention.

Summary

Several mechanisms could explain the preventive action of probiotics against colorectal cancer onset. They include: alteration of the intestinal microflora; inactivation of cancerogenic compounds; competition with putrefactive and pathogenic microbiota; improvement of the host’s immune response; anti-proliferative effects via regulation of apoptosis and cell differentiation; fermentation of undigested food; inhibition of tyrosine kinase signaling pathways.

Metabolism and biomarkers of heterocyclic aromatic amines in molecular epidemiology studies: lessons learned from aromatic amines

Aromatic amines and heterocyclic aromatic amines (HAAs) are structurally related classes of carcinogens that are formed during the combustion of tobacco or during the high-temperature cooking of meats. Both classes of procarcinogens undergo metabolic activation by N-hydroxylation of the exocyclic amine group to produce a common proposed intermediate, the arylnitrenium ion, which is the critical metabolite implicated in toxicity and DNA damage. However, the biochemistry and chemical properties of these compounds are distinct, and different biomarkers of aromatic amines and HAAs have been developed for human biomonitoring studies. Hemoglobin adducts have been extensively used as biomarkers to monitor occupational and environmental exposures to a number of aromatic amines; however, HAAs do not form hemoglobin adducts at appreciable levels, and other biomarkers have been sought. A number of epidemiologic studies that have investigated dietary consumption of well-done meat in relation to various tumor sites reported a positive association between cancer risk and well-done meat consumption, although some studies have shown no associations between well-done meat and cancer risk. A major limiting factor in most epidemiological studies is the uncertainty in quantitative estimates of chronic exposure to HAAs, and thus, the association of HAAs formed in cooked meat and cancer risk has been difficult to establish. There is a critical need to establish long-term biomarkers of HAAs that can be implemented in molecular epidemioIogy studies. In this review, we highlight and contrast the biochemistry of several prototypical carcinogenic aromatic amines and HAAs to which humans are chronically exposed. The biochemical properties and the impact of polymorphisms of the major xenobiotic-metabolizing enzymes on the biological effects of these chemicals are examined. Lastly, the analytical approaches that have been successfully employed to biomonitor aromatic amines and HAAs, and emerging biomarkers of HAAs that may be implemented in molecular epidemiology studies are discussed.

The struggle within: microbial influences on colorectal cancer

Recently, an unprecedented effort has been directed at understanding the interplay between chronic inflammation and development of cancer, with the case of inflammatory bowel disease (IBD)-associated colorectal cancer at the forefront of this research endeavor. The last decade has been particularly fertile, with the discovery of numerous innovative paradigms linking various inflammatory, proliferative, and innate and adaptive immune signaling pathways to the development of colorectal cancer. Because of the preponderant role of the intestinal microbiota in the initiation and progression of IBD, recent efforts have been directed at understanding the relationship between bacteria and colorectal cancer. The microbiota and its collective genome, the microbiome, form a diverse and complex ecological community that profoundly impacts intestinal homeostasis and disease states. This review will discuss the differential influence of the microbiota on the development of IBD-associated colorectal cancer and highlight the role of innate immune sensor-dependent as well as -independent mechanisms in this pathology.

Flavonoids inhibit the genotoxicity of hydrogen peroxide (H(2)O(2)) and of the food mutagen 2-amino-3-methylimadazo[4,5-f]-quinoline (IQ) in lymphocytes from patients with inflammatory bowel disease (IBD)

Inflammatory bowel disease (IBD) including Crohn's disease (CD) and ulcerative colitis (UC) is a chronic inflammatory gastrointestinal autoimmune condition with an inappropriate immune response. We investigated DNA damage induced in vitro in lymphocytes from IBD patients caused by oxidative stress through H(2)O(2) and 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and whether the plant flavonoids, quercetin and epicatechin, found in fruits, tea and soybeans could effectively reduce such stress. Lymphocytes from IBD patients and healthy volunteers were treated with 50 microg/ml H(2)O(2) or IQ in the presence of quercetin (0-250 microg/ml) or epicatechin (0-100 microg/ml). Flavonoid supplementation (250 microM quercetin or 100 microM epicatechin) caused an overall significant decrease of induced DNA damage resulting in a 48.6% (P < 0.001) reduction of H(2)O(2)-induced and a 43% (P < 0.001) reduction of IQ-induced DNA damage within the patient groups; for the control groups, reductions in DNA damage were 35.2 and 57.1%, respectively (both, P < 0.001). There was less induced DNA damage within lymphocytes from UC patients compared to CD patients for both series of experiments (H(2)O(2) and quercetin, IQ and epicatechin). In conclusion, flavonoids dramatically reduced oxidative stress in vitro in lymphocytes from IBD patients and healthy individuals. Thus, flavonoids could be very effective in the treatment of oxidative stress and encouraged in the diet of IBD patients.

Ability of probiotic Lactobacillus casei DN 114001 to bind or/and metabolise heterocyclic aromatic amines in vitro

INTRODUCTION

Heterocyclic aromatic amines (HCA) are compounds with high mutagenic potential, formed when meat is cooked at high temperatures of 150-300 degrees C. These compounds contribute to development of colon and gastric cancer. Western diet provides a lot of HCA and influences the available substrates for the intestinal microbiota which can activate HCA to direct acting mutagens. On the other hand, lactic acid bacteria existing in the colon and ingested with food including probiotics, may exert an anti-carcinogenic action, but the mechanism is still poorly understood.

MATERIALS AND METHODS

In the present study we determined the ability of probiotic Lactobacillus casei DN 114001 (Actimel strain) to metabolise or adsorb three HCA: IQ, MelQx and PhIP in vitro. Lactobacilli were cultivated in MRS and in a modified MRS broth with reduced concentrations of nitrogen and carbon (MRS NC), with addition of 25 mug/ml of IQ, MelQx or PhIP. Their concentration after cultivation with L. casei DN 114001 was measured with high-performance liquid chromatography and the genotoxicity was evaluated by the alkaline comet assay.

RESULTS AND CONCLUSIONS

It was measured, that after 24 h cultivation in MRS (cell density was 10(9) cfu/ml), rapid decrease of IQ and PhIP (98-99%) was observed, and the peaks on chromatograms were almost completely reduced. In case of MeIQx the decrease was about 27%. In a modified MRS broth (cell density was 10(8) cfu/ml), the ability to decrease HCA concentration during prolonged cultivation (to 168 h) depended on the growth phase of bacteria, and it was about 51.5% for IQ and at about 11.2% for MeIQx. Non-growing cells (in phosphate buffer), could reduce the content of IQ and PhIP from 72 h to the end of incubation. L. casei DN 114001 reduced genotoxicity of HCA (IQ from 46 to 48%; MeIQx from 35 to 65% and PhIP from 32 to 81%), and the degree depended on the incubation time, cell growth and the medium used. It may suggest that bacteria can metabolise or adsorb HCA.

The use of cultured primary bovine colon epithelial cells as a screening model to detect genotoxic effects of heterocyclic aromatic amines in the comet assay

Isolated epithelial cells from the bovine colon were maintained in dividing monolayer cultures and used as a model system for colon tissue in in vitro toxicological studies. The cytotoxic effects of the heterocyclic aromatic amines 2-amino-3-methylimidazo [4,5-f]quinoline (IQ) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhiP) were investigated in these cells and IC(50) values were determined by inhibition of neutral red uptake into the cultured cells. Although PhiP was not cytotoxic up to concentrations of 500 microM, IQ was cytotoxic above 300 microM. The induction of DNA strand breaks in cultured bovine epithelial colon cells was determined using the alkaline single-cell gel electrophoresis (Comet assay) technique, and subsequently, the DNA damage was used as a determinant of genotoxic effects of the heterocyclic aromatic amines in order to establish this system for detection of adverse effects of chemicals in a model system for the colon. In the absence of an external enzymatic metabolizing system (S9 mix) both amines did not induce DNA strand breaks. When S9 mix was used, PhiP induced DNA strand breaks above 10 microM whereas IQ did not show any significant effect at 300 microM. This cell culture system was found to be a useful screening system for testing of compounds that are considered to affect colonic tissue.

Overview of experimental data on reduction of colorectal cancer risk by inulin-type fructans

Colorectal cancer is related to diet, lifestyle, physical inactivity, and obesity. The responsible carcinogens cause mutations or enhance cell growth. Inulin-type fructans may counteract the effects via their gut flora-mediated fermentation products in vitro and in vivo. Important products formed by fermentation of inulin-type fructans with human gut flora are short-chain fatty acids. Of these, butyrate and propionate inhibit growth of colon tumor cells and histone deacetylases. Butyrate also causes apoptosis, reduces metastasis in colon cell lines, and protects from genotoxic carcinogens by enhancing expression of enzymes involved in detoxification. Fermentation supernatants of inulin have similar growth-inhibitory effects on colon adenoma and carcinoma cells and induce histone hyperacetylation by inhibiting histone deacetylases. In animal models inulin-type fructans prevent and retard colorectal carcinogenesis. Several studies reported the reduction of chemically induced preneoplastic lesions or tumors in the colon of rodents treated with inulin-type fructans. The human intervention study (SYNCAN project) sought to provide the experimental evidence for risk reduction by inulin-type fructans in humans. One group of polypectomized people at high risk for colon cancer and another of colon cancer volunteers after curative resection were given a synbiotic preparation. There were clear functional effects of the synbiotic because numerous different cancer risk markers were favorably altered. In conclusion, there is considerable experimental evidence that inulin modulates parameters of colon cancer risks in human colon cells, in animals, and in a human intervention trial. The involved mechanisms possibly include reduction of exposure to risk factors and suppression of tumor cell survival.

Metabolism of Maillard reaction products by the human gut microbiota--implications for health

The human colonic microbiota imparts metabolic versatility on the colon, interacts at many levels in healthy intestinal and systemic metabolism, and plays protective roles in chronic disease and acute infection. Colonic bacterial metabolism is largely dependant on dietary residues from the upper gut. Carbohydrates, resistant to digestion, drive colonic bacterial fermentation and the resulting end products are considered beneficial. Many colonic species ferment proteins but the end products are not always beneficial and include toxic compounds, such as amines and phenols. Most components of a typical Western diet are heat processed. The Maillard reaction, involving food protein and sugar, is a complex network of reactions occurring during thermal processing. The resultant modified protein resists digestion in the small intestine but is available for colonic bacterial fermentation. Little is known about the fate of the modified protein but some Maillard reaction products (MRP) are biologically active by, e. g. altering bacterial population levels within the colon or, upon absorption, interacting with human disease mechanisms by induction of inflammatory responses. This review presents current understanding of the interactions between MRP and intestinal bacteria. Recent scientific advances offering the possibility of elucidating the consequences of microbe-MRP interactions within the gut are discussed.

Suppressive effect of Clostridium perfringens-produced heat-stable substance(s) on proliferation of human colon adenocarcinoma HT29 cells in culture

Clostridium perfringens has been regarded as one of the intestinal bacteria increasing colon cancer risk. In previous studies, we have shown that the oral administration of C. perfringens culture medium can inhibit the mutagen-induced formation of pre-neoplastic lesions in rat colon, thus proposing the existence of factor(s) preventing colon tumorigenesis in this culture medium. However, the properties of effective factor(s) and the mechanism of this inhibitory action still remain to be investigated. Then, the effect of C. perfringens culture medium on human colon adenocarcinoma HT29 cells was examined. The exposure of HT29 cells to C. perfringens culture medium was found to suppress the proliferation of these cells probably through the reduction of immediate early gene egr-1 expression. These observations suggest that C. perfringens culture medium has a cytostatic action on colon tumor cells, which may be responsible for the prevention of pre-neoplastic formation in rat colon.

Diet and cancer: assessing the risk

Globally, colorectal cancer (CRC) is a leading cause of mortality from malignant disease. Case-control and cohort studies provide strong support for a role of diet in the aetiology of CRC. However to establish causal relationships and to identify more precisely the dietary components involved, intervention studies in human subjects are required. Cancer is an impractical endpoint in terms of numbers, cost, study duration and ethical considerations. Consequently, intermediate biomarkers of the disease are required. This review aims to provide an overview of the intermediate endpoints available for the study of CRC, particularly non-invasive faecal biomarkers. Examples of their use in dietary intervention studies are given.

Impact of bacteria in dairy products and of the intestinal microflora on the genotoxic and carcinogenic effects of heterocyclic aromatic amines

This article gives a short overview on the present state of knowledge of the effects of the intestinal microflora on the health hazards of heterocyclic aromatic amines (HAs). Results of single cell gel electrophoresis assays with conventional, germ free and human flora associated rats indicate that the presence of intestinal microorganisms strongly enhances the induction of DNA-damage in colon and liver cells by IQ. Furthermore, it was found that supplementation of the feed with Lactobacilli attenuates the induction of colon cancer by this same amine. These recent findings suggest that the intestinal microflora and lactic acid bacilli in dairy products strongly affect the health risks of HAs. Nevertheless, most previous experiments with HAs focused on the involvement of mammalian enzymes in the biotransformation of these compounds and only a few articles are available which concern interactions of bacteria with HAs. Some of these studies suggested that the formation of directly mutagenic hydroxy-metabolites of the amines by fecal bacteria might be an important activation pathway but it turned out that the hydroxy-derivative of IQ is not genotoxic in mammalian cells and does not cause colon cancer in laboratory rodents. There is some evidence that hydrolysis of HA-metabolites by bacterial ss-glucuronidase might play a role in the activation of HAs but experimental data are scarce and no firm conclusions can be drawn at present. The most important detoxification mechanism appears to be the direct binding of the HAs to the cell walls of certain bacterial strains contained in fermented foods. It was shown that these effects do also take place under physiologically relevant conditions. Overall, it seems that intestinal bacteria play a key role in the activation and detoxification of HAs which has been an area of research long ignored. The elucidation of these mechanisms may enable the development of biomarkers for colon cancer risk and nutritional strategies of protection.

Increased expression of multidrug resistance related proteins Pgp, MRP1, and LRP/MVP occurs early in colorectal carcinogenesis

AIM

To analyse the expression of multidrug resistance (MDR) related proteins at different steps in colorectal carcinogenesis.

METHODS

The presence of three MDR related proteins (Pgp, MRP1, and LRP/MVP) was studied by means of immunohistochemistry in normal, adenomatous, and malignant colorectal epithelium. Formaldehyde fixed, paraffin embedded tissue sections of 17 samples of colorectal tissue were used (normal mucosa, n = 4; adjacent mucosa, n = 5; adenoma, n = 5; carcinoma, n = 3).

RESULTS

For all three proteins, expression was found in the surface epithelium and the upper parts of the crypts in normal colon. In the adenomas, staining was seen along the complete length of the crypts. In the carcinomas analysed, all epithelium showed positive staining. Mucosa adjacent to either carcinoma or adenoma showed staining patterns mostly resembling those of normal mucosa, but sometimes some extension of staining was seen along the crypt.

CONCLUSIONS

These proteins already show increased expression in the adenoma stage. In the absence of adequate mucin production in adenomas, MDR related proteins could be an important factor in protecting the epithelium against further environmentally induced genetic damage. This could be one of the reasons why only about 5% of colorectal adenomas will actually progress to carcinomas.

Effect of intestinal bacteria on formation of azoxymethane-induced aberrant crypt foci in the rat colon

The effect of intestinal bacteria on formation of azoxymethane (AOM)-induced aberrant crypt foci (ACF) and DNA adducts in the rat colon was investigated. Male Sprague-Dawley rats were administered cultures of Lactobacillus acidophilus, Bifidobacterium adolescentis, Bacteroides fragilis, Escherichia coli and Clostridium perfringens for five weeks and given injections of AOM at 15 mg/kg body weight at the first and second weeks. The number of ACF five weeks after the start of the experiment was decreased in the rats treated with the cultures or culture supernatants of L. acidophilus and C. perfringens. The half-life of O6-methylguanine (O6-meG) in the L. acidophilus group was shorter than that in the GAM broth group. The half-life of 7-methylguanine did not differ among the groups. These results suggest that the metabolite(s) of L. acidophilus and C. perfringens inhibit(s) the ACF formation in rats treated with AOM and that the inhibitory effect of L. acidophilus is due to the enhanced removal of O6-meG from the colon mucosal DNA.

Tocopherols and the etiology of colon cancer

Colorectal cancer is the second most common cause of cancer deaths in the United States for both sexes. Considerable evidence suggests that the risk of this cancer is increased by the mutagenic actions of free radicals, which are produced during oxidation reactions. Dietary factors, the intestinal flora (bacteria), and endogenously produced metabolites contribute to the production of free radicals in the colon. Dietary antioxidants, such as vitamin E, should reduce the levels of these harmful oxidation products. In the absence of vitamin E, polyunsaturated fats can be oxidized in the colon to produce mutagens, such as lipid hydroperoxides and malondialdehyde. Furthermore, fecal bacteria can generate a high flux of reactive oxygen species (e.g., the superoxide radical [O2*-]) at the surface of the intestinal lumen, and inflammatory cells in close proximity to the colon can produce reactive nitrogen species (e.g., nitrogen dioxide [NO2]). Increasing evidence suggests that the different chemical (e.g., alpha- and gamma-tocopherol) and stereochemical (e.g., RRR- and all-racemic-alpha-tocopherol) forms of vitamin E have distinct biologic potencies, pharmacokinetics, and different abilities to prevent neoplastic transformation. This review considers and evaluates recent studies relating vitamin E and oxidative stress to colon cancer, emphasizing the distinct roles of alpha- and gamma-tocopherols. In addition, recent findings on the antioxidant/pro-oxidant status of the digesta (ingested food) are discussed with respect to the use of antioxidants in chemo-prevention trials for colon cancer.

Metabolism of food-derived heterocyclic amines in nonhuman primates

During the cooking of meats, several highly mutagenic heterocyclic amines (HCAs) are produced. Three HCAs, IQ, MeIQx, and PhIP have been under study for carcinogenicity in cynomolgus monkeys, and to date, IQ has been shown to be a potent hepatocarcinogen. Concomitantly, the metabolic processing of these HCAs has been examined. Metabolism studies show that the potent hepatocarcinogenicity of IQ is associated with the in vivo metabolic activation of IQ via N-hydroxylation and the formation of DNA adducts. In monkeys undergoing carcinogen bioassay with IQ, N-hydroxylation was confirmed by the presence of the N-hydroxy-N-glucuronide conjugate of IQ in urine. The N-hydroxylation of IQ appears to be carried out largely by hepatic CYP3A4 and/or CYP2C9/10, and not by CYP1A2, an isoform not expressed in liver of this species. Notably MeIQx is poorly activated in cynomolgus monkeys and lacks the potency of IQ to induce hepatocellular carcinoma after a 5-year dosing period. The poor activation of MeIQx appears to be due to the lack of constitutive expression of CYP1A2 and an inability of other cytochromes P450, such as CYP3A4 and CYP2C9/10, to N-hydroxylate the quinoxalines. MeIQx is detoxified in monkeys largely by conjugation with glucuronide at the N-1 position. Although the carcinogenicity of PhIP is not yet known, the metabolic data suggest that PhIP will be carcinogenic in this species. PhIP is metabolically activated in vivo in monkeys by N-hydroxylation, as discerned by the presence of the N-hydroxy-N-glucuronide conjugate in urine, bile, and plasma. PhIP also produces DNA adducts that are widely distributed in tissues. The results from these studies support the importance of N-hydroxylation in the carcinogenicity of HCAs in nonhuman primates and by analogy, the importance of this metabolic activation step in the possible carcinogenicity of dietary HCAs in humans.

The secretory phospholipase A2 gene is a candidate for the Mom1 locus, a major modifier of ApcMin-induced intestinal neoplasia

Mutations in the APC gene are responsible for various familial and sporadic colorectal cancers. Min mice carry a dominant mutation in the homolog of the Apc gene and develop multiple adenomas throughout their small and large intestine. Quantitative trait loci studies have identified a locus, Mom1, which maps to the distal region of chromosome 4, that dramatically modifies Min-induced tumor number. We report here the identification of a candidate gene for Mom1. The gene for secretory type II phospholipase A2 (Pla2s) maps to the same region that contains Mom1 and displays 100% concordance between allele type and tumor susceptibility. Expression and sequence analysis revealed that Mom1 susceptible strains are most likely null for Pla2s activity. Our results indicate that Pla2s acts as a novel gene that modifies polyp number by altering the cellular microenvironment within the intestinal crypt.

A factor in the increased risk of colorectal cancer due to ingestion of animal fat is inhibition of colon epithelial cell glutathione S-transferase, an enzyme that detoxifies mutagens

Dietary animal fat increases the risk of colorectal cancer. A factor in the increased risk is hypothesised to result from the inhibition of isoforms of a colonic epithelial cell enzyme that detoxifies genotoxins, glutathione S-transferase, by one of the major secondary bile acids produced in the colon by fat digestion, lithocholic acid. The inhibition allows mutagens to persist in colonic epithelial cells while proliferation is stimulated by secondary bile acids, with a concomitant greater frequency of neoplasia-associated mutations than when proliferation is stimulated in the absence of the mutagens. Elements in the hypothesis include the ability of relatively low concentrations of lithocholic acid to inhibit isoforms of glutathione S-transferase found in colon epithelial cells, entry of lithocholic acid into the epithelial cells, and the correlation of neoplasia-associated colon pathology with high levels of lithocholic acid in fecal water. Higher pH values in the colonic stream are identified as exacerbating the effects of lithocholic acid by increasing its solubility. Lithocholic acid is suggested to be more inhibitory to glutathione S-transferase than the other major colonic secondary bile acid, deoxycholic acid, on the basis of inhibition-structure relationships.

Laboratory and epidemiologic studies of fecapentaenes

Fecapentaenes are a class of conjugated ether lipids which have been identified as the major component of human fecal mutagenicity in the Ames Salmonella mutagenesis assay. Human epidemiologic data have indicated that most healthy North American populations eating a western diet do excrete detectable levels of fecapentaenes. Excreted fecapentaene levels seem to reflect levels throughout the colonic lumen, and levels vary characteristically between individuals. Those individuals found to excrete high levels of fecapentaene appear, based on limited data, to be at decreased risk of colorectal neoplasia. Carcinogenicity studies in rats and mice have been predominantly negative, however, increased tumor incidence in mice exposed to fecapentaenes as neonates has recently been reported. Fecapentaenes are direct-acting genotoxins, which may react with DNA through free radical mechanisms, and/or aldehyde formation. Mechanisms by which fecapentaene-induced DNA damage may mediate carcinogenesis are discussed.