Regulation of serum amyloid A3 (SAA3) in mouse colonic epithelium and adipose tissue by the intestinal microbiota

Toll-like receptor signalling in the intestinal epithelium: how bacterial recognition shapes intestinal function

A single layer of epithelial cells lines the small and large intestines and functions as a barrier between commensal bacteria and the rest of the body. Ligation of Toll-like receptors (TLRs) on intestinal epithelial cells by bacterial products promotes epithelial cell proliferation, secretion of IgA into the gut lumen and expression of antimicrobial peptides. As described in this Review, this establishes a microorganism-induced programme of epithelial cell homeostasis and repair in the intestine. Dysregulation of this process can result in chronic inflammatory and over-exuberant repair responses, and it is associated with the development of colon cancer. Thus, dysregulated TLR signalling by intestinal epithelial cells may explain how colonic bacteria and inflammation promote colorectal cancer.

Premetastatic milieu explained by TLR4 agonist-mediated homeostatic inflammation

Accumulating evidence suggests that Toll-like receptor 4 (TLR4), a sensor for danger signals, is expressed not only in immune cells, but also in resident epithelial cells, and appears to participate in tissue homeostasis. To explain the premetastatic microenvironment created by the newly discovered endogenous TLR4 ligands, I propose a hypothesis of homeostatic inflammation that includes the classical danger hypothesis.

Microbial regulation of SAA3 expression in mouse colon and adipose tissue

Recently, we demonstrated that colonic and adipose expression of SAA3 was modulated by the gut microbiota and Toll-like receptor signaling in mice. We observed that SAA3 was expressed by colonic epithelial cells and that its expression was induced in a murine colonocyte cell line following lipopolysaccharide stimulation and nuclear NFκB translocation. In this addendum, we extend this initial study and suggest that SAA3 (1) resembles human SAA1 both in amino acid homology and tissue distribution, (2) appears to have autocrine or paracrine effects rather than endocrine, and (3) binds to bacteria within the gastrointestinal tract. Although speculative, these observations raise the possibility that SAA3 may promote local inflammation in adipose tissue that affects insulin signaling and also function as an antimicrobial agent in the colon.

Obesity and the human microbiome

PURPOSE OF REVIEW

Obesity was once rare, but the last few decades have seen a rapid expansion of the proportion of obese individuals worldwide. Recent work has shown obesity to be associated with a shift in the representation of the dominant phyla of bacteria in the gut, both in humans and animal models. This review summarizes the latest research into the association between microbial ecology and host adiposity, and the mechanisms by which microbes in the gut may mediate host metabolism in the context of obesity.

RECENT FINDINGS

Studies of the effect of excess body fat on the abundances of different bacteria taxa in the gut generally show alterations in the gastrointestinal microbiota, and changes during weight loss. The gastrointestinal microbiota have been shown to impact insulin resistance, inflammation, and adiposity via interactions with epithelial and endocrine cells.

SUMMARY

Large-scale alterations of the gut microbiota and its microbiome (gene content) are associated with obesity and are responsive to weight loss. Gut microbes can impact host metabolism via signaling pathways in the gut, with effects on inflammation, insulin resistance, and deposition of energy in fat stores. Restoration of the gut microbiota to a healthy state may ameliorate the conditions associated with obesity and help maintain a healthy weight.

Effect of overweight on gastrointestinal microbiology and immunology: correlation with blood biomarkers

A cross-sectional study was carried out in order to compare intestinal microbiological and immunological biomarkers with blood glucose and lipids, satiety-related hormones and inflammatory biomarkers characterising differences between obese and normal weight subjects. Faecal and blood samples were obtained from twenty obese subjects with an average BMI of 32·9 kg/m2and twenty normal weight subjects with an average BMI of 23·3 kg/m2. Blood insulin, TAG and leptin were significantly elevated, whereas concentrations of HDL and ghrelin were significantly decreased in the obese subjects. Inflammatory status in the obese subjects was characterised by a trend for elevated blood C-reactive protein (CRP;P = 0·06) and IL-6 (P = 0·02). The faecal microbial composition differed between the groups; less sulphate-reducing bacteria (P = 0·05) and a trend for lessBacteroides(P = 0·07) were measured for overweight subjects. Furthermore, an inverse correlation was demonstrated between faecalBacteroideslevels and waist circumference (P = 0·05). The faecal microbial metabolites differed between the groups; increased concentrations of branched-chain fatty acids, phenolics, valeric acid, di- and hydroxy acids were described in the obese subjects. No differences between the measured intestinal inflammatory biomarkers were detected. However, systemic inflammation (CRP and IL-6) was correlated with the faecal concentrations of phenolics and lactic acid (P < 0·05 and 0·05, andP < 0·01 and 0·05, respectively). In summary, weight-related differences were observed both in the intestinal microbial composition and its activity. The role of intestinal signals, such as phenolics and lactic acid in the development of weight-related problems, needs to be studied further.

Coordinated regulation of the metabolome and lipidome at the host-microbial interface

The creative use of gnotobiotic animals, coupled with the development of modern metagenomic sequencing platforms and metabolomic profiling of biospecimens, has bestowed new insight into the remarkably intricate interface between the host mammal and its resident microbiota. As mutual benefactors, each partner exhibits evidence of adaptation: the host provides a hospitable habitat, giving consideration to its own species of origin, diet, genotype, geographical location, presence or absence of disease, and use of medications; the microbiota, in turn, configures its constituency, collective genome (microbiome), transcriptome, and metabolome to optimally suit its ecological niche. In this review, we discuss the mechanisms through which the gut microbiota and its host collaborate to regulate lipid metabolism, thereby influencing the metabolic response to nutrient intake and ultimately, the development of obesity and associated diseases such as lipotoxicity. These studies therefore demonstrate that the gut microbiota is an "environmental" influence whose synergistic interdependence with its host strongly suggests that we are in fact "supraorganisms."