The effect of long-term high-fat diet feeding on intestinal health of aged vervet monkeys

Abstract

The consumption of Westernised high-fat diets (HFDs) dysregulates intestinal barrier components which may lead to subclinical systemic and tissue inflammation implicated in type 2 diabetes (T2D) development. It is essential to understand the intestinal barrier‘s role in developing T2D. This may aid in the identification of potential therapeutic and preventative targets for intestinal barrier defects and downstream metabolic impairments. This study investigated intestinal barrier function and inflammation in intestinal tissues of vervet monkeys (Chlorocebus aethiops) towards the development of T2D. The study made use of intestinal tissues (duodenum, jejunum, ileum and colon) and serum that was previously collected from nine vervet monkeys that were maintained on a maize-based control diet (MD) (n=3) and an HFD (n=6), respectively for 15 years. Serum was used to assess glycaemic and lipogram parameters. Haematoxylin and Eosin (H&E) staining was used for general morphological assessment and immunohistochemistry (IHC) was used to assess the expression of intestinal immunity marker, immunoglobulin-A-positive (IgA+) cells of the duodenum, jejunum, ileum and colon. IHC was also used to assess the expression of intestinal barrier integrity marker, occludin in the ileum and colon. Western blot analysis was used to assess the expression levels of markers that are involved in the synthesis of pro-inflammatory mediators, extracellular signal-regulated kinase 1/2 (ERK1/2) and p38 in the ileum and colon. The expression levels of leaky gut biomarkers in serum: lipopolysaccharide-binding protein (LBP) and cluster of differentiation 14 (CD14) were assessed using respective enzyme-linked immunosorbent assay (ELISA) kits. The results revealed that the HFD did not induce significant changes in fasting blood glucose nor glycated haemoglobin levels of the vervet monkeys when compared to MD-fed vervet monkeys. There were no significant changes in triglyceride between HFD-fed and MD-fed vervet monkeys. Furthermore, total cholesterol levels were marginally increased (p=0.071), high-density lipoprotein cholesterol and low-density lipoprotein cholesterol levels were significantly increased (p=0.048 and p=0.017 respectively) in HFD-fed vervet monkeys as compared to MD- fed monkeys. H&E staining revealed that an HFD did not affect the morphology of the ileum and colon of vervet monkeys as evidenced by no significant changes in the villus length and crypt depth respectively when compared to MD-fed vervet monkeys. Immunohistochemical analysis revealed that an HFD significantly increased (p=0.046) the population of IgA+ cells in the duodenum of vervet monkeys when compared to MD-fed vervet monkeys. However, no significant changes in the population of IgA+ cells were demonstrated in the jejunum, ileum and colon between HFD-fed and MD- fed vervet monkeys. HFD-fed vervet monkeys did not demonstrate significant changes in the expression of occludin in the ileum and colon when compared to MD-fed vervet monkeys. Western blot analysis revealed that there were no significant changes in the expression of ERK1/2 and p38 in the ileum and colon between HFD-fed and MD-fed vervet monkeys. There were no significant differences in serum levels of LBP and CD14 assessed by ELISA between HFD-fed and MD-fed vervet monkeys. In conclusion, this study demonstrated that an HFD dysregulated lipid metabolism in HFD-fed vervet monkeys, suggesting a probable predisposition to developing metabolic disease. In addition, the increased population of IgA+ cells in the duodenum of HFD-fed vervet monkeys suggests dysregulated mucosal immunity. However, further research is warranted to elucidate underlying mechanisms.