Non-digestible carbohydrates, the WNT signalling pathway and bowel cancer risk

Abstract

Epidemiological and experimental evidence suggests that non-digestible carbohydrates (NDCs) e.g. resistant starch (RS) are protective against colorectal cancer (CRC), resulting primarily from the production of the short-chain fatty acid (SCFA) butyrate. The WNT signalling pathway is central to the maintenance of homeostasis within the large bowel through regulation of physiological processes and is frequently aberrantly activated in CRCs. Butyrate has been shown to positively modulate WNT signalling, affecting functional outcomes such as apoptosis and proliferation and, therefore, protecting against CRC. To investigate the molecular mechanisms through which NDCs may reduce CRC risk in humans, we undertook a double-blind, randomised, controlled trial (The DISC Study) in which 75 healthy participants were supplemented with two NDCs: RS (Hi-maize® 260) and polydextrose (PD) (Litesse® Ultra™) for 50 days using a 2x2 factorial design. Colorectal mucosal biopsies were collected before and at the end of intervention and used to quantify expression of WNT pathway-related genes and of BAX and BCL-2 (two regulators of apoptosis) and to assess colonic crypt cell proliferation. WNT signalling pathway genes were also investigated in an additional 38 participants at higher risk of CRC because of quiescent ulcerative colitis or a history of adenomatous polyps. RS supplementation significantly reduced expression of CTNNB1 (p=0.045), encoding β-catenin (a key component of the WNT pathway), and c-MYC (p=0.037), suggesting a reduction in WNT signalling in these participants. RS and/or PD significantly reduced expression of two antagonists of WNT signalling, SFRP1 and SFRP2, suggesting an increase in WNT activity. RS and PD did not affect SFRP1 methylation or expression of miRNAs that may target this gene. RS increased total colonic crypt cell proliferation (p=0.030) but did not alter the proportion of mitotic cells in the top half of the crypt (a marker of crypt health). Participants at higher risk of CRC had increased expression of c-JUN (p=0.046) and WNT11 (p=0.040) and increased SFRP1 methylation. The BAX to BCL-2 ratio was lower in higher-risk participants, suggesting a reduction in BCL-2 ii family-mediated apoptosis. Unexpectedly, crypt cell proliferation was also reduced in higher-risk participants (p=0.009). These findings suggest that the increased cell proliferation with RS supplementation may have resulted from induction of the WNT pathway. However, these effects were not mediated via alterations in SFRP1 methylation or expression of miRNAs. This study has also shown that WNT signalling is aberrantly active in the macroscopically-normal mucosa of people at higher risk of CRC.