Studies into the relationship between GPCR43 and BuA-induced effects on colorectal cancer.

Abstract

Colorectal cancer (CRC) is a major problem in affluent countries worldwide. In Australia it is the second most commonly diagnosed malignancy with approximately 13,000 new cases diagnosed each year. This disease is also the leading cause of cancer related death in Australia with approximately 4,500 fatalities each year. Epidemiological studies have shown geographical variation in the incidence of disease, with diet considered to be a key contributing factor to CRC risk. In particular, diets high in fibre and low in fat have been demonstrated to reduce the risk of developing CRC. Fibre is heterogeneous in nature and can be categorised into different subtypes. Resistant starch is a component of fibre which remains largely intact throughout the gastrointestinal tract until it reaches the colon. Here it undergoes bacterial fermentation to produce the short chain fatty acids (SCFAs) acetate, propionate and butyrate (BuA). Each of the SCFAs are bioactive in the colon, with the most active being BuA. The beneficial effects of fibre have been linked to BuA’s ability to induce colon cancer cell differentiation, reduce proliferation and initiate apoptosis. Interestingly, in normal cells BuA is utilised as the preferential energy source and has been shown to promote proliferation. With an apparent “paradoxical effect” on normal and cancerous cells BuA has been the subject of much investigation as a potential anticancer agent. Despite numerous studies investigating BuA actions, the exact biological mechanisms remain largely undefined. This thesis explored a possible mechanism for BuA-induced apoptosis and inhibition of proliferation. In 2003, two publications provided evidence that SCFAs, including BuA, were ligands to two members of a previously orphan family of G-protein coupled receptors (GPCRs); GPCR41 and 43. Of the two receptors BuA had the strongest effect on GPCR43. Consequently this thesis investigated the possibility that BuA acts to decrease CRC proliferation and induce apoptosis by binding to and activating GPCR43 on CRC cells. It was hypothesised that GPCR43 acted as a “BuA sensor” on the surface of the cell to mediate the effects of BuA. This experimental work utilised PCR, Q-PCR, measures of apoptosis, proliferation and differentiation and RNAi knockdown. The key areas of investigation included: (1) Determining if GPCR43 was present on a range of CRC cell lines with a cell line to represent adenocarcinoma, carcinoma and metastatic stage of disease. (2) Investigating the expression of GPCR43 with manipulated nutrient media and different levels of cell confluence. (3) Exploring GPCR43 expression in normal and malignant human patient biopsies. (4) Determining if the inhibition of G-protein function using inhibitors influenced BuAinduced changes to apoptosis and proliferation. (5) Using RNAi, investigating the effect that GPCR43 knockdown would have on BuA-induced changes to proliferation and apoptosis. The key findings from this work included: (1) Presence of GPCR43 on some but not all CRC cell lines. (2) Modulation of GPCR43 expression with exposure to BuA and altered glucose concentrations in the media. (3) An influence of G-protein inhibition on BuA-induced apoptosis but not proliferation in some cell lines. (4) GPCR43 knockdown using RNAi indicated that GPCR43 is not exclusively required for BuA to regulate apoptosis and proliferation. The results from this work indicate that GPCR43 is not likely to exclusively mediate BuA’s effects, but opens up new areas of research into the exact role of GPCR43 on CRC cells.