Anti-Inflammatory and Anti-Oxidative Activity of Indole-3-Acetic Acid Involves Induction of HO-1 and Neutralization of Free Radicals in RAW264.7 Cells

The cellular and molecular mechanisms by which indole-3-acetic acid (IAA), a tryptophan-derived metabolite from gut microbiota, attenuates inflammation and oxidative stress has not been fully elucidated. The present study was to unearth the protective effect and underlying mechanism of IAA against lipopolysaccharide (LPS)-induced inflammatory response and free radical generation in RAW264.7 macrophages. IAA significantly ameliorated LPS-induced expression of interleukin-1β (IL-1β), interleukin-6 (IL-6), and monocyte chemoattractant protein-1 (MCP-1) as well as generation of reactive oxidative species (ROS) and nitric oxide (NO). LPS-triggered nuclear translocation of nuclear factor kappa B (NF-κB) p65 was mitigated by IAA treatment. Further, an up-regulation of heme oxygenase-1 (HO-1) was observed in IAA-treated cells in dose-dependent manner under both normal and LPS-stimulated condition. Interference of HO-1 activity by tin protoporphyrin IX (SnPP) impeded the alleviative effects of IAA on expression of IL-1β and IL-6 induced by LPS, whereas demonstrated no effect on its suppression of ROS and NO production. This result suggests a HO-1-dependent anti-inflammatory effect of IAA and its direct scavenging action on free radicals. Treatment with CH-223191, a specific antagonist of Aryl Hydrocarbon Receptor (AhR), showed no significant effects on the beneficial role of IAA against inflammation and free radical generation. In summary, our findings indicate that IAA alleviates LPS-elicited inflammatory response and free radical generation in RAW264.7 macrophages by induction of HO-1 and direct neutralization of free radicals, a mechanism independent of AhR.