5-HT_2Bserotonin receptor agonist BW723C86 shapes the macrophage gene profile via AhR and impairs monocyte-to-osteoclast differentiation

Peripheral serotonin (5-HT) exacerbates or limits inflammatory pathologies through interaction with seven types of 5-HT receptors (5-HT1-7). As central regulators of inflammation, macrophages are critical targets of 5-HT, which promotes their anti-inflammatory and pro-fibrotic polarization primarily via the 5-HT7-Protein Kinase A (PKA) axis. However, anti-inflammatory human macrophages are also characterized by the expression of 5-HT2B, an off-target of anesthetics, anti-parkinsonian drugs and Selective Serotonin Reuptake Inhibitors (SSRI) that contributes to 5-HT-mediated pathologies. Since 5-HT2Bprevents mononuclear phagocyte degeneration in amyotrophic lateral sclerosis and modulates motility of murine microglial processes, we sought to determine the functional and transcriptional consequences of 5-HT2Bactivation in human macrophages. Ligation of 5-HT2Bby the 5-HT2B-specific agonist BW723C86, which exhibits antidepressant- and anxiolytic-like effects in animal models, significantly modified the cytokine profile and the transcriptional signature in macrophages. Importantly, 5-HT2Bagonist-induced transcriptional changes were partly mediated through activation of the Aryl hydrocarbon Receptor (AhR), a ligand-dependent transcription factor that regulates immune responses and the biological responses to xenobiotics. Besides, BW723C86 triggered transcriptional effects that could not be abrogated by 5-HT2Bantagonists and impaired monocyte-to-osteoclast differentiation by affecting the expression of negative (IRF8) and positive (PRDM1) regulators of osteoclastogenesis. Therefore, our results demonstrate the existence of a functional 5-HT2B-AhR axis in human macrophages and indicate that the commonly used 5-HT2Bagonist BW723C86 exhibits 5-HT2B-independent effects. The 5-HT2B-AhR link extends the range of signaling pathways initiated upon 5-HT receptor engagement and identifies a point of convergence for endogenous and exogenous agents with ability to modulate inflammatory responses.KEY POINTS-The serotonin receptor 5-HT2Bmodifies the human macrophage transcriptome through activation of the Aryl Hydrocarbon Receptor.-BW723C86, an agonist used for 5-HT2Bactivationin vivo, exerts 5-HT2B-independent effects and limits monocyte osteoclastogenic potential.