Microbiota-Derived Metabolites, Indole-3-aldehyde and Indole-3-acetic Acid, Differentially Modulate Innate Cytokines and Stromal Remodeling Processes Associated with Autoimmune Arthritis

Abstract

Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic inflammation of the synovial joints. Inflammation, new blood vessel formation (angiogenesis) and bone resorption (osteoclastogenesis) are three key processes involved in the joint damage and deformities of arthritis. Various gut microbiota-derived metabolites are implicated in RA pathogenesis. However, there is barely any information about the impact of two such metabolites, indole-3-aldehyde (IAld) and indole-3-acetic acid (I3AA), on arthritis-related processes. We conducted a comparative analysis of IAld and I3AA using established cell-based models to understand how they might influence RA pathogenesis. Although structurally similar, the bioactivities of these two metabolites were profoundly different. IAld but not I3AA, inhibited the expression of pro-inflammatory cytokines (IL-1β and IL-6) in RAW 264.7 (RAW) cells stimulated with heat-killed M. tuberculosis sonicate (Mtb) and lipopolysaccharide (LPS). IAld also exhibited pro-angiogenic activity and pro-osteoclastogenic activity. In contrast, I3AA exhibited anti-angiogenic activity on endothelial cell tube formation but had no effect on osteoclastogenesis. Both IAld and I3AA have been proposed as aryl hydrocarbon receptor (AhR) agonists. Use of CH-223191, an inhibitor of the AhR, suppressed the anti-angiogenic activity of I3AA but failed to mitigate the effects of IAld. Further investigation of the anti-inflammatory activities of IAld and I3AA in LPS-treated RAW cells indicated that inhibition of MyD88-dependent activation of NF-κB and MAPK pathways was not likely involved. Our results suggest that the relative bioavailability of these indole derivatives may differentially impact RA progression and possibly other diseases that share similar cellular processes.

Proteomic Characterization of the Cellular Effects of AhR Activation by Microbial Tryptophan Catabolites in Endotoxin-Activated Human Macrophages

“…In contrast, IAld is rapidly degraded by aldehyde dehydrogenase [ 46 , 47 ]. In addition, the indoles might not be as potent AhR agonists as BaP, but they are relevant molecules which can affect microbiome homeostasis and host immune response [ 48 ].…”

“…However, stimulation with both molecules was followed by the same ratio of decreased AKT phosphorylation [ 27 ]. Even the structurally similar tryptophan metabolites IAld and I3AA affected the immune response in RAW 264.7 macrophages activated with LPS in different ways [ 48 ]. Whereas treatment with 500 µM IAld for 2 h was followed by a distinct reduction of IL-1β, IL-6, and TNF, only slight changes of IL-1β and TNF expression were observed after stimulation with the same amount of I3AA [ 48 ].…”

“…Even the structurally similar tryptophan metabolites IAld and I3AA affected the immune response in RAW 264.7 macrophages activated with LPS in different ways [ 48 ]. Whereas treatment with 500 µM IAld for 2 h was followed by a distinct reduction of IL-1β, IL-6, and TNF, only slight changes of IL-1β and TNF expression were observed after stimulation with the same amount of I3AA [ 48 ]. In this study, a 10-fold lower concentration (50 µM) of the respective indole metabolites was used and had cellular effects in human macrophages already.…”

“…Observing ligand-specific effects, potential AhR independent mechanisms should be considered, likewise. In a recent study, Langan et al demonstrated differing effects for IAld and I3AA [ 48 ]. IAld I3AA inhibited the expression of IL1b and IL6 in an AhR dependent manner, whereas I3AA showed AhR-independent anti-angiogenic activity in cell-based models of rheumatoid arthritis [ 48 ].…”

Multiple roles of haem in cystathionine β-synthase activity: implications for hemin and other therapies of acute hepatic porphyria

“…For example, by lowering aromatic L-amino acid decarboxylase activity in the gut, a B 6 deficiency can decrease serotonin and melatonin synthesis and undermine their peripheral activity, such as modulation of gastrointestinal and immune function [77][78][79]. With microbiota, the B 6 deficiency can decrease production of tryptamine, indole, indol-3-ylpyruvate and their metabolites potentially undermining, among others, their intestinal protection, protective aryl hydrocarbon receptor ligand activity and suppression of production of proinflammatory cytokines and chemokines [80][81][82]. Other than the occurrence of a proinflammatory environment in AIP, it is currently unknown if any of the above effects of PLP deficiency is related to AIP symptoms.…”