The Microbially Derived Metabolite Indole Attenuates Obesity Associated Inflammatory Processes in Adipocytes and Macrophages

Abstract

The human gastrointestinal (GI) tract is colonized by ~10^14 bacteria belonging to ~1,000 species that are collectively termed the intestinal microbiota. Recent studies show that the intestinal microbiota impacts a wide range of functions in the GI tract including development of the immune system, defense against pathogens, and inflammation. Indole is derived from dietary tryptophan from microbial enzymatic activity. Previous work from our laboratory identified indole attenuates indicators of inflammation, increases tight junction resistance and increases colonization resistance in intestinal epithelial cells. This work identifies the effects of indole extend to both adipocytes and macrophages beyond intestinal epithelial cells. Indole attenuates LPS-mediated induction of TNF-α production in RAW264.7 murine macrophages. Additionally, indole also attenuates LPS-mediated MCP-1 expression in 3T3-L1 adipocytes. The inhibition of MCP-1 expression in adipocytes was mirrored by a significant attenuation of bone marrow derived macrophage migration toward MCP-1 and adipocyte conditioned media. Indole correspondingly inhibits macrophage stimulated media induction of MCP-1 in adipocytes. Therefore indole interferes with paracrine signaling between macrophages and adipocytes. Indole is also exhibits agonist and antagonist activity for the AhR. However, the indole-mediated decreases in macrophage chemotaxis towards MCP-1 and in LPS induction of TNF-α production in BMDMs are independent of the AhR. Further, utilizing a novel indole-protein conjugate, we show the effects of indole do not require uptake of indole into the cytoplasm. In addition, this work also demonstrates that indole may signal through the mTOR pathway in macrophages. These studies suggest indole exerts effects on inflammatory processes by altering the mTOR signaling pathway and suggest an alternative mechanism of action to the recently identified effects of indole on the AhR pathway.