Biomarkers for colon cancer : Applications in human and rat studies
Abstract: Colon cancer is one of the major causes of cancer in industrialized countries, and it is caused by a combination of hereditary, environmental and dietary factors and lack of physical activity. Of these factors, diet is one of the most important. To increase our knowledge about the diet-colon cancer link, it is important to conduct human intervention studies using good cancer markers. Presently, the most reliable marker is occurrence of polyps in the colon. As this method is invasive, time demanding and very costly, it is important to have access to early/intermediate biomarkers for testing hypotheses relating diet and cancer. Approximately 25 biomarkers for colon cancer risk are presently available, for colon mucosa, faeces and blood. Faecal water is the aqueous phase of faeces, and this phase is believed to mediate many of the effects of diet on colon cancer. The composition of faecal water is not fully understood, but it probably contains both tumour promoters and anti-cancer agents. The overall aim with this thesis was to further characterize a selection of early/intermediate biomarkers and to assess the colon cancer preventative effects of; polyethylene glycol (PEG), probiotic bacteria, prebiotic carbohydrates, and natural cyclooxygenase-2 (COX-2) inhibitors. In the first study, we investigated whether three selected biomarkers differed between a high risk (adenoma patients) and a low risk group for colon cancer. Colonic cell proliferation induced by faecal water was significantly higher in the high risk group compared to the low risk group, and may therefore be a useful biomarker for future intervention studies. Two additional biomarkers; preneoplastic lesions (ACF) and the inflammatory protein GMP were used in a rat study to study the cancer preventative effects of the demulcent PEG. PEG significantly reduced ACF formation and faecal inflammation measured by GMP. In order to evaluate the colon cancer preventive properties of synbiotics (probiotics and prebiotics), a double-blind intervention study was carried out in humans. Patients with polyps or a history of cancer were recruited, and a large number of biomarkers were assessed. The synbiotic supplements significantly altered the colonic microflora in a favorable way. The intervention also significantly reduced colorectal proliferation and DNA damage, and the capacity of colonic contents (faecal water) to induce necrosis and DNA damage in cells. Earlier studies have indicated that faecal water can influence COX-2, which is a marker for colon cancer. A small pilot study with vegetarians was carried out to assess if natural COX-2 inhibitors were present in faecal water. Natural COX-2 inhibitors were present in human faecal water, and further characterisation indicated that the COX-2 inhibitors are polar and water soluble, and that they act in synergy. Some data also indicated a possible role for phenolic compounds in these effects. In conclusion, our work will aid in the selection of appropriate biomarkers in future human intervention trials to assess the effect of dietary components on colon cancer risk. PEG and synbiotics are two agents that may be useful in reducing risk for tumours in the colon. Lastly, natural COX-2 inhibitors are present in human faeces, and may also be a target for dietary recommendations in the future.
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