Metabolic investigation of dietary impact on colorectal cancer risk

Abstract

Colorectal cancer (CRC) is the third most commonly diagnosed cancer and the second leading cause of cancer death worldwide. Yet there is large geographical variation in incidence, with CRC often referred to as a ‘Westernised’ disease. A large body of both epidemiological and experimental evidence has linked the consumption of a diet high in fat and biological protein to increased CRC risk, whereas an inverse association has been demonstrated for high levels of fibre consumption. Moreover, it is theorised that diet’s impact on CRC risk is mediated through colonic microbial metabolism and the subsequent production of either pro- or anti-carcinogenic metabolites such as secondary bile acids and short chain fatty acids (SCFA), respectively. In the second and third chapters of this thesis, ultra high-performance mass-spectrometry (UPLC-MS) was used to investigate diet associated changes to faecal and urinary metabolites of participants in a Dietary Exchange Study. African American participants, who typically eat a high-fat, low-fibre, Western diet swapped diets for two weeks with rural African participants, who typically eat a high-fibre, low-fat diet, and vice versa. Previously published data demonstrated that Westernisation of the diet led to a striking increase in biomarkers of CRC risk in rural Africans, yet Africanisation of the diet led to decreased CRC biomarkers in African Americans. In Chapter 2, global profiling revealed six metabolites associated with changes to diet and CRC risk. Acylcarnitines increased in both faecal and urinary samples in response to the adoption of a Western diet and with further mechanistic evidence, could serve as biomarkers of diet associated increase in CRC risk. In Chapter 3, analysis of faecal samples using an UPLC-MS bile acid profiling method demonstrated increases in secondary bile acids associated with the consumption of a high-fat, low-fibre diet. However, in contrast to this trend, 3-ketocholanic acid (3-KCA) a derivative of carcinogenic lithocholic acid (LCA), increased with the consumption of a high-fibre, low-fat diet. This highlighted a potential LCA detoxification mechanism. In Chapter 4, this was investigated further. Firstly, both faecal microbiota and selected microbial species were shown to have the capacity to produce 3-KCA. This was a novel finding. Secondly, the carcinogenic potential of 3-KCA compared to that of LCA was investigated using a HCT116 cell line. 3-KCA was shown to be significantly less cytotoxic than LCA, and preliminary results also showed a trend towards reduced genotoxicity. Although further experiments, using additional cell lines and animal models, will be required to validate these results, these initial data imply that 3-KCA may be less carcinogenic than LCA. Taken together, these data shine a spotlight on the potential of synbiotic intervention, harnessing both probiotic species with 3-KCA producing capacity and the substrates which sustain them, in the detoxification of colonic LCA, and thereby in the reduction of CRC risk in high-risk populations. Accordingly, Chapter 5 set out to explore the potential of prebiotic intervention for CRC risk reduction in Alaska Native peoples, who suffer the highest incidence rates of CRC globally. Urinary and faecal samples from the first 20 participants of the ongoing ‘Fibre to reduce colon cancer in Alaska Native peoples’ study were analysed using Nuclear Magnetic Resonance (NMR) analysis. Although global faecal and urinary metabolic profiles were not significantly changed by the intervention, it remains to be seen whether this will remain the case when the remaining samples are analysed on study completion. Despite this, these data demonstrate the potential of NMR-based profiling in the non-invasive and accurate detection of CRC biomarkers. Collectively, the results presented in this thesis highlight how microbiota manipulation can potentially be harnessed to reduced CRC risk and how NMR and UPLC-MS can be used to assess associated metabolomic changes in a non-invasive manner.