Oral Nutrition Modulates the Intestinal Barrier Following Cytotoxic Therapy Via the Microbiome

Reduced oral intake (ROI) is common in patients that have received cytotoxic therapy for hematologic malignancies. Here we examine the interplay between nutrition, intestinal bacteria and intestinal barrier function, focusing on preclinical mouse models.

By 3 days following 9 Gy of total body irradiation (TBI), individually-housed C57BL/6 mice were observed to have an approximately 40-60% reduction in oral intake (Fig A), and 3 days later displayed marked changes in the microbiome by 16S deep sequencing, including an expansion of Bacteroides thetaiotaomicron (Fig B). These mice also had lost roughly 20% of their body weight (Fig C), and showed compromise of their intestinal barrier assayed by oral challenge with FITC-dextran (Fig M). We hypothesized that the ROI could be a driving factor behind the microbiome changes. To assess this, we limited the access of unirradiated mice to 2 grams of normal chow a day for 7 days (an approximately 50% reduction in oral intake) with unlimited water. Microbiome profiling by 16S sequencing showed many changes that were similar to those seen in irradiated mice, including an expansion of Bacteroides thetaiotaomicron (Fig D). Because Bacteroides thetaiotaomicron has been shown to utilize glycans derived from colonic mucin, we evaluated the consumption of porcine gastric mucin by intestinal bacteria in vitro with a colorimetric assay that quantifies polysaccharides. We found that following ROI, mice harbored intestinal bacteria that were significantly more efficient at utilizing mucin glycans (Fig E). Corroborating this, a histological characterization of the colonic mucus thickness following fixation with Carnoy's solution showed that after a week of ROI, unirradiated mice developed significant thinning of the colonic mucus layer (Fig F), though intestinal barrier function otherwise remained intact as assayed by FITC-dextran gavage (data not shown).

We asked how ROI could favor mucolytic bacteria and hypothesized that ROI could reduce bacterial fermentation of dietary carbohydrates which largely results in production of short-chain fatty acids. Evaluating the pH of the colonic lumen showed that ROI led to a significant elevation in pH (Fig G). Ion-chromatography mass spectrometry of colonic stool samples showed a reduction in acetate, propionate, butyrate, and lactate following ROI, and interestingly an increase in succinate (Fig I). Corroborating this, RNA sequencing of intestinal bacteria following ROI demonstrated a downregulation of phosphoenolpyruvate carboxykinase, a metabolic enzyme that in Bacteroides species has been previously been found to be associated with conversion of succinate to propionate (Fig H).

Finally, we evaluated in our ROI model the impact of administration of oral sugars by supplementing a well-absorbed sugar (glucose), and a poorly-absorbed prebiotic sugar in the drinking water of mice. We found that compared with glucose, supplementation with the prebiotic more effectively acidified the colonic lumen of normal mice (Fig J), and completely prevented thinning of the mucus layer in mice undergoing ROI (Fig K). Additionally, in mice following 9 Gy of total body irradiation, daily administration of the prebiotic was highly effective in preventing weight loss (Fig L) and nearly completely abrogated compromise of intestinal barrier function (Fig M).

Conclusions:

Reduced oral intake following cytotoxic therapy contributes to thinning of the colonic mucus layer, an effect that appears to be independent of the cytotoxic effects on the host and is rather mediated by changes in the intestinal microbiome, including reduced metabolism of organic acids. Strategies to restore colonic acidity with prebiotic sugars may successfully target this phenomenon with potential clinical benefits, particularly in preventing disruption to the intestinal barrier.

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