Interference of colonic luminal compounds with the mucosal butyrate metabolism in ulcerative colitis.
Ulcerative colitis (UC) is an inflammatory bowel disease (IBD) which is characterised by chronic, relapsing inflammation of the colonic mucosa. The most common clinical symptoms include bloody diarrhoea, abdominal pain, fatigue and weight loss. Conventional therapeutic options are mainly targeted at reducing the inflammatory process but are only partially successful and associated with a range of side effects. The exact patho-physiological processes underlying this disease remain elusive, although increasing evidence supports the involvement of aberrant immune responses to environmental and gut microbial triggers in genetically susceptible hosts. The colonic microbiota has an imbalanced composition in UC and has been suggested to play a key role in the initiation and progression of disease activity. Through bacterial fermentation, an extensive range of colonic luminal compounds are produced that are in close interaction with the colonic mucosa and might influence intestinal health.
In the present PhD project we focused on the role of the colonic compounds in the pathophysiology of UC. Their composition was characterised in faecal samples from UC patients and compared with that from healthy controls. In addition, their capacity to affect various parameters of gut function was investigated and discriminating compounds were identified. Furthermore, the safety of a novel butyrate-producing strain was investigated in healthy subjects.
In the first part of this project we showed that the induction of inflammation in an epithelial cell line downregulates the uptake and oxidation of butyrate, which is the main energy-providing process for colonocytes (Chapter 3). This confirms the hypothesis that the impaired colonic butyrate metabolism in UC patients is consequential to the inflammation. Simultaneous incubation with butyrate counteracted the reduced butyrate oxidation, indicating that increasing luminal butyrate levels might have therapeutic potential in UC.
In the second part we characterised the microbial dysbiosis in UC by a reduced biodiversity, decreased abundances of butyrate-producing species and increased levels of some opportunistic pathogens such as Helicobacter and Escherichia/Shigella. Analysis of the faecal metabolome revealed an aberrant composition of colonic compounds compared to healthy controls, including decreased levels of the short-, medium- and long-chain fatty acids (SCFAs, MCFAs and LCFAs) whereas some amino acids and bile acids were increased in UC (Chapter 4).
Subsequently, we investigated the functional impact of this microbial dysbiosis by evaluating whether the colonic compounds from UC patients influenced physiological processes relevant to the UC pathogenesis. Faecal water from UC patients reduced the gene expression of the first enzyme in the butyrate oxidation pathway in colonic cells (Chapter 5), provoked increased epithelial cytotoxicity (Chapter 6), decreased the epithelial barrier resistance (Chapter 7) and increased the production of inflammatory cytokines in human peripheral blood mononuclear cells (Chapter 8) in comparison to faecal water from HC. This indicates that the altered composition of colonic compounds in UC patients has a more pronounced detrimental effect on gut functioning compared to HC, suggesting they have the potential to directly trigger, sustain or aggravate intestinal inflammation in UC patients. Colonic compounds that were consistently associated with changes in these gut function parameters included SCFAs, MCFAs and LCFAs (beneficial) together with ethanol, dimethyl disulfide and amino acids (detrimental). Altering their luminal concentrations in UC, which can be achieved by dietary adjustments promoting carbohydrate fermentation, may be a promising strategy to improve the colonic health in UC.
An alternative strategy to increase luminal butyrate concentrations and target microbial dysbiosis is the use of butyrate-producing bacteria. In the third part of this project we conducted a placebo-controlled cross-over exploratory phase 1 trial with daily administration of the butyrate-producer Butyricicoccus (B.) pullicaecorum 25-3T in 30 healthy subjects. The intervention proved to be safe and well-tolerated, paving the way for its use in clinical settings to test its therapeutic effects (Chapter 9). Furthermore, B. pullicaecorum intervention increased total microbial richness, indicating the promotion of a more stable and resilient intestinal ecosystem.
In conclusion, the results obtained in this PhD add to the current understanding of the pathophysiology of UC. Microbial dysbiosis leads to an altered production of colonic compounds which adversely affect gut health in UC and may contribute to the initiation, propagation or aggravation of the inflammatory process. Dietary changes which promote carbohydrate over protein fermentation, resulting in increased microbial SCFA production, and the limitation of alcohol consumption may entail gut health benefits for UC patients.