Interkingdom signaling is established in the gastrointestinal tract in that human hormones trigger responses in bacteria; here, we show that the corollary is true, that a specific bacterial signal, indole, is recognized as a beneficial signal in intestinal epithelial cells. Our prior work has shown that indole, secreted by commensal Escherichia coli and detected in human feces, reduces pathogenic E. coli chemotaxis, motility, and attachment to epithelial cells. However, the effect of indole on intestinal epithelial cells is not known. Because intestinal epithelial cells are likely to be exposed continuously to indole, we hypothesized that indole may be beneficial for these cells, and investigated changes in gene expression with the human enterocyte cell line HCT-8 upon exposure to indole. Exposure to physiologically relevant amounts of indole increased expression of genes involved in strengthening the mucosal barrier and mucin production, which were consistent with an increase in the transepithelial resistance of HCT-8 cells. Indole also decreased TNF-α-mediated activation of NF-κB, expression of the proinflammatory chemokine IL-8, and the attachment of pathogenic E. coli to HCT-8 cells, as well as increased expression of the antiinflammatory cytokine IL-10. The changes in transepithelial resistance and NF-κB activation were specific to indole: other indole-like molecules did not elicit a similar response. Our results are similar to those observed with probiotic strains and suggest that indole could be important in the intestinal epithelial cells response to gastrointestinal tract pathogens.host-pathogen interactions | interkingdom signaling | probiotics T he human gastrointestinal (GI) tract is rich in a diverse range of signaling molecules. A wide range of bacterial signals (e.g., autoinducer-2, autoinducer-3, and indole) (1) are produced by the ∼10 14 nonpathogenic commensal bacteria that coexist with host cells in the GI tract, and neuroendocrine hormones (e.g., norepinephrine and dopamine) are also synthesized in situ in the GI tract via the enteric nervous system. The close proximity of bacteria and the host cells in the GI tract, as well as the high local concentrations of the signals they secrete, has led to a signalcentric paradigm wherein GI tract signals are considered to be important mediators of homeostasis and infections through intrakingdom (i.e., recognition of bacterial signals by other bacteria) and/or interkingdom (i.e., recognition of host signals by bacteria and vice versa) signaling and communication.

The gut microbiota plays a significant role in the progression of fatty liver disease; however, the mediators and their mechanisms remain to be elucidated. Comparing metabolite profile differences between germ-free and conventionally raised mice against differences between mice fed a low- and high-fat diet (HFD), we identified tryptamine and indole-3-acetate (I3A) as metabolites that depend on the microbiota and are depleted under a HFD. Both metabolites reduced fatty-acid- and LPS-stimulated production of pro-inflammatory cytokines in macrophages and inhibited the migration of cells toward a chemokine, with I3A exhibiting greater potency. In hepatocytes, I3A attenuated inflammatory responses under lipid loading and reduced the expression of fatty acid synthase and sterol regulatory element-binding protein-1c. These effects were abrogated in the presence of an aryl-hydrocarbon receptor (AhR) antagonist, indicating that the effects are AhR dependent. Our results suggest that gut microbiota could influence inflammatory responses in the liver through metabolites engaging host receptors.

The tryptophan metabolites indole, indole-3-acetate, and tryptamine were identified in mouse cecal extracts and fecal pellets by mass spectrometry. The aryl hydrocarbon receptor (AHR) agonist and antagonist activities of these microbiotaderived compounds were investigated in CaCo-2 intestinal cells as a model for understanding their interactions with colonic tissue, which is highly aryl hydrocarbon (Ah)-responsive. Activation of Ah-responsive genes demonstrated that tryptamine and indole 3-acetate were AHR agonists, whereas indole was an AHR antagonist that inhibited TCDD (2,3,7,8-tetrachlorodibenzo-pdioxin)-induced CYP1A1 expression. In contrast, the tryptophan metabolites exhibited minimal anti-inflammatory activities, whereas TCDD decreased phorbol ester-induced CXCR4 [chemokine (C-X-C motif) receptor 4] gene expression, and this response was AHR dependent. These results demonstrate that the tryptophan metabolites indole, tryptamine, and indole-3-acetate modulate AHR-mediated responses in CaCo-2 cells, and concentrations of indole that exhibit AHR antagonist activity (100-250 mM) are detected in the intestinal microbiome.