Dietary Fiber Treatment Corrects the Composition of Gut Microbiota, Promotes SCFA Production, and Suppresses Colon Carcinogenesis

Abstract

Epidemiological studies propose a protective role for dietary fiber in colon cancer (CRC). One possible mechanism of fiber is its fermentation property in the gut and ability to change microbiota composition and function. Here, we investigate the role of a dietary fiber mixture in polyposis and elucidate potential mechanisms using TS4Cre×cAPC^l°^x468 mice. Stool microbiota profiling was performed, while functional prediction was done using PICRUSt. Stool short-chain fatty acid (SCFA) metabolites were measured. Histone acetylation and expression of SCFA butyrate receptor were assessed. We found that SCFA-producing bacteria were lower in the polyposis mice, suggesting a decline in the fermentation product of dietary fibers with polyposis. Next, a high fiber diet was given to polyposis mice, which significantly increased SCFA-producing bacteria as well as SCFA levels. This was associated with an increase in SCFA butyrate receptor and a significant decrease in polyposis. In conclusion, we found polyposis to be associated with dysbiotic microbiota characterized by a decline in SCFA-producing bacteria, which was targetable by high fiber treatment, leading to an increase in SCFA levels and amelioration of polyposis. The prebiotic activity of fiber, promoting beneficial bacteria, could be the key mechanism for the protective effects of fiber on colon carcinogenesis. SCFA-promoting fermentable fibers are a promising dietary intervention to prevent CRC.

Keywords: CRC; SCFA; butyrate; dietary fiber; microbiota.

Figure 1

(A,B) Significantly different microbial profiles between TS4/APC (TS4Cre × cAPC^l°^×468 mice) chow-fed and B6 (C57BL/6J control mice) chow-fed mice feces: (A) Principal coordinate analysis (PCoA) profile of microbial diversity (genus) across all samples, with the percentage of variation explained by PC1 and PC2 indicated in the axis; (B) Stacked bars of significantly different (FDR-p ˂ 0.05) rarefied sequences of microbial taxa (relative abundance percentages of total bacteria at phylum, family, and genus) between mice groups.

Figure 2

(A–C) Significantly different microbial profiles between TS4/APC and B6 chow-fed mice feces: (A) Stacked histograms depicting 90% of the rarefied sequences of microbial taxa (relative abundance percentages of total bacteria at the genus) between mice groups; (B) Firmicutes-to-Bacteroidetes ratio and; (C) Short-chain-fatty-acid (SCFA) bacterial taxa both significantly lower in TS4/APC chow-fed mice.

Figure 3

Alpha diversity is significantly lower in TS4/APC high fiber-fed mice compared to chow-fed mice. Plots show the Shannon (p = 0.023), Richness (p = 0.033), and Evenness (p = 0.033) values between mice groups at the genus level. Two-tailed unpaired Independent t-test was used to analyze differences for parametric data satisfying test assumptions.

Figure 4

(A–C) Significantly different microbial profiles between TS4/APC chow-fed and high fiber-fed mice feces: (A) Principal coordinate analysis (PCoA) profile of microbial diversity (genus) across all samples, with the percentage of variation explained by PC1 and PC2 indicated in the axis; (B) Stacked bars of significantly different (p ˂ 0.05) rarefied sequences of microbial taxa (relative abundance percentages of total bacteria at genus) between mice groups; (C) Stacked histograms depicting 90% of the rarefied sequences of microbial taxa (relative abundance percentages of total bacteria at the genus) between mice groups.

Figure 5

Functional predictions for fecal microbiomes of TS4/APC high fiber-fed and chow-fed mice. Significantly different KEGG (Kyoto Encyclopedia of Genes and Genomes) pathways (p < 0.05) with a greater ratio mean abundance in TS4/APC high fiber-fed mice relative to chow-fed mice, as inferred using PICRUSt (Phylogenetic Investigation of Communities by Reconstruction of Unobserved States) analysis of fecal microbiomes.

Figure 6

Significant changes in all short-chain fatty acids’ (SCFAs) metabolite levels, when examined in the entire cohort of TS4/APC chow-fed and high fiber-fed mice. Total SCFA concentration (MWU; p ˂ 0.0001), acetic acid (MWU; p ˂ 0.0001), propionic acid (p = 0.0011), and butyric acid (p = 0.0034) were all significantly increased in fecal samples of high fiber-fed mice compared to chow-fed mice. Mice (n = 10) per group.

Figure 7

(A–C) Effect of the high fiber diet on polyposis in TS4/APC mice (n = 5 per group): (A) Number of polyps was significantly reduced in high fiber-fed mice compared to chow-fed mice; (B) Left: Representative of histone acetylation blot, as assessed by H3K9ac antibody, on cecum tissue of high fiber-fed and chow-fed mice; Right: Western blot Expression of H3K9ac was normalized to the expression of Actin and was compared between the two groups; (C) Left: Representative of GPR109a blot using cecum tissue of high fiber-fed and chow-fed mice; Right: Western blot Expression of GPR109a, after normalization to Actin expression, increased by fiber treatment (n = 5 per group); asterisk represents p < 0.05.