Trace amine associated receptor 1 (TAAR1) expression and modulation of inflammatory cytokine production in mouse bone marrow-derived macrophages: a novel mechanism for inflammation in ulcerative colitis

Context: Tissue resident macrophages and peripherally infiltrating macrophages play a prominent role in maintaining homeostasis in the gastrointestinal tract (GIT), though aberrant activation is implicated in inflammatory conditions, including ulcerative colitis (UC). Recent metabolomic studies indicate that tyramine (TYR) is elevated in the stool of patients with UC. TYR activates the mammalian trace amine associated receptor 1 (TAAR1). Our previous work identified TAAR1 expression in mixed populations of immune cells, whereas a limited number of other studies have identified TAAR1-dependent effects in cytokine secretion and gene expression in T-cells and B-cells. Objective: To investigate whether TAAR1 may serve as a novel target for an anti-inflammatory therapeutic in UC, we explored TAAR1 expression in mouse bone marrow-derived macrophages (BMDMs), and its upregulation and activation in response to LPS and TYR. Results: Here, we demonstrate for the first time that TAAR1 is expressed in BMDM and undergoes agonist-induced upregulation. Additionally, TYR elicits significant increases in inflammatory cytokine gene expression in non-polarized and LPS-polarized BMDM, and the TAAR1 antagonist EPPTB inhibits the TYR-mediated upregulation of TAAR1 and inflammatory cytokine gene expression in BMDM. Conclusions: Our data suggest that TAAR1 is a mediator of macrophage inflammation and a potential therapeutic target to attenuate UC symptomology.