The epidemiology and molecular biology of colorectal cancer are reviewed with a view to understanding their interrelationship. Risk factors for colorectal neoplasia include a positive family history, meat consumption, smoking, and alcohol consumption. Important inverse associations exist with vegetables, nonsteroidal anti-inflammatory drugs (NSAIDs), hormone replacement therapy, and physical activity. There are several molecular pathways to colorectal cancer, especially the APC (adenomatous polyposis coli)-beta-catenin-Tcf (T-cell factor; a transcriptional activator) pathway and the pathway involving abnormalities of DNA mismatch repair. These are important, both in inherited syndromes (familial adenomatous polyposis [FAP] and hereditary nonpolyposis colorectal cancer [HNPCC], respectively) and in sporadic cancers. Other less well defined pathways exist. Expression of key genes in any of these pathways may be lost by inherited or acquired mutation or by hypermethylation. The roles of several of the environmental exposures in the molecular pathways either are established (e.g., inhibition of cyclooxygenase-2 by NSAIDs) or are suggested (e.g., meat and tobacco smoke as sources of specific blood-borne carcinogens; vegetables as a source of folate, antioxidants, and inducers of detoxifying enzymes). The roles of other factors (e.g., physical activity) remain obscure even when the epidemiology is quite consistent. There is also evidence that some metabolic pathways, e.g., those involving folate and heterocyclic amines, may be modified by polymorphisms in relevant genes, e.g., MTHFR (methylenetetrahydrofolate reductase) and NAT1 (N-acetyltransferase 1) and NAT2. There is at least some evidence that the general host metabolic state can provide a milieu that enhances or reduces the likelihood of cancer progression. Understanding the roles of environmental exposures and host susceptibilities in molecular pathways has implications for screening, treatment, surveillance, and prevention.

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Colorectal Cancer: Molecules and Populations

Human alteration of the nitrogen cycle has resulted in steadily accumulating nitrate in our water resources. The U.S. maximum contaminant level and World Health Organization guidelines for nitrate in drinking water were promulgated to protect infants from developing methemoglobinemia, an acute condition. Some scientists have recently suggested that the regulatory limit for nitrate is overly conservative; however, they have not thoroughly considered chronic health outcomes. In August 2004, a symposium on drinking-water nitrate and health was held at the International Society for Environmental Epidemiology meeting to evaluate nitrate exposures and associated health effects in relation to the current regulatory limit. The contribution of drinking-water nitrate toward endogenous formation of N-nitroso compounds was evaluated with a focus toward identifying subpopulations with increased rates of nitrosation. Adverse health effects may be the result of a complex interaction of the amount of nitrate ingested, the concomitant ingestion of nitrosation cofactors and precursors, and specific medical conditions that increase nitrosation. Workshop participants concluded that more experimental studies are needed and that a particularly fruitful approach may be to conduct epidemiologic studies among susceptible subgroups with increased endogenous nitrosation. The few epidemiologic studies that have evaluated intake of nitrosation precursors and/or nitrosation inhibitors have observed elevated risks for colon cancer and neural tube defects associated with drinking-water nitrate concentrations below the regulatory limit. The role of drinking-water nitrate exposure as a risk factor for specific cancers, reproductive outcomes, and other chronic health effects must be studied more thoroughly before changes to the regulatory level for nitrate in drinking water can be considered.

Workgroup report: Drinking-water nitrate and health--recent findings and research needs.

Colorectal cancer is one of the so-called westernized diseases and the second leading cause of cancer death worldwide. On the basis of global epidemiological and scientific studies, evidence suggests that the risk of colorectal cancer is increased by processed and unprocessed meat consumption but suppressed by fibre, and that food composition affects colonic health and cancer risk via its effects on colonic microbial metabolism. The gut microbiota can ferment complex dietary residues that are resistant to digestion by enteric enzymes. This process provides energy for the microbiota but culminates in the release of short-chain fatty acids including butyrate, which are utilized for the metabolic needs of the colon and the body. Butyrate has a remarkable array of colonic health-promoting and antineoplastic properties: it is the preferred energy source for colonocytes, it maintains mucosal integrity and it suppresses inflammation and carcinogenesis through effects on immunity, gene expression and epigenetic modulation. Protein residues and fat-stimulated bile acids are also metabolized by the microbiota to inflammatory and/or carcinogenic metabolites, which increase the risk of neoplastic progression. This Review will discuss the mechanisms behind these microbial metabolite effects, which could be modified by diet to achieve the objective of preventing colorectal cancer in Western societies.

Diet, microorganisms and their metabolites, and colon cancer

Objective: To assess the epidemiological evidence on diet and cancer and make public health recommendations. Design: Review of published studies, concentrating on recent systematic reviews, meta-analyses and large prospective studies. Conclusions and recommendations: Overweight/obesity increases the risk for cancers of the oesophagus (adenocarcinoma), colorectum, breast (postmenopausal), endometrium and kidney; body weight should be maintained in the body mass index range of 18.5-25 kg/m 2 , and weight gain in adulthood avoided. Alcohol causes cancers of the oral cavity, pharynx, oesophagus and liver, and a small increase in the risk for breast cancer; if consumed, alcohol intake should not exceed 2 units/d. Aflatoxin in foods causes liver cancer, although its importance in the absence of hepatitis virus infections is not clear; exposure to aflatoxin in foods should be minimised. Chinese-style salted fish increases the risk for nasopharyngeal cancer, particularly if eaten during childhood, and should be eaten only in moderation. Fruits and vegetables probably reduce the risk for cancers of the oral cavity, oesophagus, stomach and colorectum, and diets should include at least 400 g/d of total fruits and vegetables. Preserved meat and red meat probably increase the risk for colorectal cancer; if eaten, consumption of these foods should be moderate. Salt preserved foods and high salt intake probably increase the risk for stomach cancer; overall consumption of salt preserved foods and salt should be moderate. Very hot drinks and foods probably increase the risk for cancers of the oral cavity, pharynx and oesophagus; drinks and foods should not be consumed when they are scalding hot. Physical activity, the main determinant of energy expenditure, reduces the risk for colorectal cancer and probably reduces the risk for breast cancer; regular physical activity should be taken.

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Diet, nutrition and the prevention of cancer

Nitrate levels in our water resources have increased in many areas of the world largely due to applications of inorganic fertilizer and animal manure in agricultural areas. The regulatory limit for nitrate in public drinking water supplies was set to protect against infant methemoglobinemia, but other health effects were not considered. Risk of specific cancers and birth defects may be increased when nitrate is ingested under conditions that increase formation of N-nitroso compounds. We previously reviewed epidemiologic studies before 2005 of nitrate intake from drinking water and cancer, adverse reproductive outcomes and other health effects. Since that review, more than 30 epidemiologic studies have evaluated drinking water nitrate and these outcomes. The most common endpoints studied were colorectal cancer, bladder, and breast cancer (three studies each), and thyroid disease (four studies). Considering all studies, the strongest evidence for a relationship between drinking water nitrate ingestion and adverse health outcomes (besides methemoglobinemia) is for colorectal cancer, thyroid disease, and neural tube defects. Many studies observed increased risk with ingestion of water nitrate levels that were below regulatory limits. Future studies of these and other health outcomes should include improved exposure assessment and accurate characterization of individual factors that affect endogenous nitrosation.

https://www.mdpi.com/1660-4601/15/7/1557/pdf

Drinking Water Nitrate and Human Health: An Updated Review

High red meat diets have been linked with risk of sporadic colorectal cancer; but their effects on mutations which occur in this cancer are unknown. G-->A transitions in K-ras occur in colorectal cancer and are characteristic of the effects of alkylating agents such as N-nitroso compounds (NOC). We studied th effect of red meat consumption on faecal NOC levels in eight male volunteers who consumed diets low or high in meat (60 or 600 g/day), as beef, lamb or pork, whilst living in a metabolic suite. Increased intake of red meat induced a significant (P<0.024) 3-fold increase from 40 + or - 7 to ab average of 113 + or - 25 microgram/day NOC, a range of exposure in faeces similar to that from tobacco-specific NOC in cigarette smoke. THe diets were isoenergetic and contained equal amounts of fat, but concentrations of heterocyclic amines were low. Faecal excretion of the promotor ammonia was significantly increased to 6.5 + or - 1.08 mmol/day. When the high red meat diets were supplemented with 20 g phytate-free wheat bran in six volunteers there was no reduction in NOC levels (mean 138 + or - 41 microgram/day NOC), but faecal weight increased. Higher starch and non-starch polysaccharide intakes reduced intraluminal cross-linking in microcapsules (r=-0.77) and reduced faecal pH (r=-0.64). In two volunteers there was no effect of 600 g white meat and fish o faecal NOC (mean low white meat diet 68 + or - 10 microgram/day, high white meat 56 + or -6 microgram/day nor on faecal nitrate, nitrite and iron. Faecal nitrite levels increased on changing from a white to red meat diet (mean high white meat diet 46 + or - 7 mg/day, high red meat diet mean 80 + or - 7 mg/day.) Increased endogenous production of NOC and precursors from increased red meat, but not white meat and fish, consumption may be relevant to the aetiology of colorectal cancer.

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Does increased endogenous formation of N-nitroso compounds in the human colon explain the association between red meat and colon cancer?

Dietary fat, protein and fibre have been shown to modulate cancer risk in humans and the present study examined the biological effects in human-flora-associated (HFA) rats of altering intake levels within the normal human range. Two control groups, one HFA and the other germfree (GF), consumed a human diet low in fat, fibre and beef for 4 weeks; three other groups consumed human diets similar except for independent 3-fold increases in fat, beef protein or fibre. After 2 weeks on the diets, magnetically recoverable microcapsules were given orally to the rats and subsequently recovered from the faeces to assess endogenous cross-linking agents. After 4 weeks, measurements were made of gut microfloral enzyme activities, hepatic activation of dietary mutagens and hepatic DNA adducts by 32P-postlabelling. Activation in vitro of the dietary mutagens 2-amino-3-methyl-3H-imidazo[4,5-f]quinoline (IQ) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) by hepatic S9, formation of endogenous hepatic DNA adducts in vivo and the beta-glucuronidase activity of caecal contents were all increased in the sequence high fat > high fibre > high beef = control. Of the two DNA adducts found in all HFA rats, only one was present in GF controls, indicating that the human gut microflora (subject to human dietary modulation) either releases a DNA-adducting product able to act outside the gastrointestinal tract, or stimulates the generation of such a product by mammalian processes. Caecal nitrate reductase activity was highest in rats fed the high beef diet, whilst entrapment of cross-linking agents was highest in those fed the high fibre diet. These results show that risk-related components of human diets interact with human gut microflora to modulate the production of endogenous DNA-adducting and cross-linking substances.

Effects of risk-associated human dietary macrocomponents on processes related to carcinogenesis in human-flora-associated (HFA) rats.

After gastrointestinal (GI) transit through rats, semipermeable bifunctional microcapsules containing polyethyleneimine (PEI) as a DNA-stimulating nucleophilic target showed physiochemical alterations consistent with PEI amine functions being intermolecularly cross-linked as by bifunctional agents. Such cross-linking both within the membrane and inside the microcapsules between core PEI and membrane PEI was simulated in vitro by glutaraldehyde, guanosine dialdehyde, 4-hydroxynonenal and fecapentaene-12, the latter two agents being known to form cyclic adducts or cross-links on DNA. These in vivo effects were demonstrated using both a radiolabel and a colorimetric label, and were attributable to both stomach and caecal sources by microcapsule recovery from the excised GI tract. By acid treatment of recovered microcapsules, cross-links formed in stomach and caecum were found to be respectively acid sensitive and acid resistant. The cross-linking effects observed were equivalent to treatment with 10 mumols glutaraldehyde but this seems a severe underestimate due both to known limited trapping of GI electrophiles by limited quantity of microcapsules and demonstrated low efficiency by glutaraldehyde in forming cross-links versus amine modifications. These results demonstrate that there are substantial concentrations of endogenous, membrane-penetrating, cross-linking/bifunctional agents in the GI tract which have significance due to the potent DNA-damaging and carcinogenic properties of such agents as a class.

Presence of endogenous cross-linking/bifunctional agents in gastrointestinal cavity as detected by transit of magnetic PEI microcapsules

Magnetically recoverable, semipermeable microcapsules have been devised for covalent entrapment of reactive substances in the intestinal cavity to biomonitor potentially DNA-damaging agents and the...

https://www.jstor.org/stable/pdfplus/3431473.pdf

Magnetic microcapsule exploration in the gastrointestinal cavity of the origins of colorectal cancer-associated DNA-damaging agents in the human diet.

Semi-permeable magnetic microcapsules previously shown able to trap gastrointestinal carcinogens and containing polyethyleneimine (PEI) were covalently labelled with [14CH3], and administered for the first time to humans (six healthy volunteers, 1.3 microCi/dose) in gelatin capsules together with radio-opaque gut transit markers (ROM), in order both to seek human endogenous cross-linking or bifunctional alkylating agents and assess gut transit features. No ill-effects were reported. Faecal ROM and 14C excretions were well correlated (r = 0.96), and net 14C recovery in faeces was 83-96%. Microcapsules were separated magnetically from faeces and 29-81% of specific labelling of microcapsules (nCi/10(6)) was found to have been removed during GI transit. Label cleavage out of these microcapsules was also found following in vitro anaerobic incubation with faecal slurries from two volunteers. On treatment with H2O2, label was removed selectively from the Fe-containing core in a dose-dependent manner. Therefore, label cleavage in vivo (not observed in rats consuming chow but found notably on consumption of low-fibre and/or high-beef human diets) is likely to arise from low mol. wt substances that give Fenton reaction producing hydroxyl radicals and oxidative demethylation. After GI transit, extensive core to membrane cross-linking in the microcapsules was found and was inversely related to faecal output. Cross-linking also was obtained to a greater extent during in vitro anaerobic incubation with faecal slurries. The GI mucosa would also be exposed to both types of agents, and several features of this microcapsule monitoring are in accord with putative risk-modulating effects. This first use of microcapsules for biomonitoring of the human GI tract thus seemed to be without hazard, and revealed extensive levels of agents likely to cause DNA damage.

Anne Ellul

Papers

Does increased endogenous formation of N-nitroso compounds in the human colon explain the association between red meat and colon cancer?

Effects of risk-associated human dietary macrocomponents on processes related to carcinogenesis in human-flora-associated (HFA) rats.

Presence of endogenous cross-linking/bifunctional agents in gastrointestinal cavity as detected by transit of magnetic PEI microcapsules

Magnetic microcapsule exploration in the gastrointestinal cavity of the origins of colorectal cancer-associated DNA-damaging agents in the human diet.

Novel detection by magnetic microcapsules in the human gastrointestinal tract of cross-linking agents and diet-dependent reactive oxygen species.