GPR84 is a member of the metabolic G protein-coupled receptor family, and its expression has been described predominantly in immune cells. GPR84 activation is involved in the inflammatory response, but the mechanisms by which it modulates inflammation have been incompletely described. In this study, we investigated GPR84 expression, activation, and function in macrophages to establish the role of the receptor during the inflammatory response. We observed that GPR84 expression in murine tissues is increased by endotoxemia, hyperglycemia, and hypercholesterolemia. Ex vivo studies revealed that GPR84 mRNA expression is increased by LPS and other pro-inflammatory molecules in different murine and human macrophage populations. Likewise, high glucose concentrations and the presence of oxidized LDL increased GPR84 expression in macrophages. Activation of the GPR84 receptor with a selective agonist, 6-(octylamino) pyrimidine-2,4(1H,3H)-dione (6-n-octylaminouracil, 6-OAU), enhanced the expression of phosphorylated Akt, p-ERK, and p65 nuclear translocation under inflammatory conditions and elevated the expression levels of the inflammatory mediators TNFα, IL-6, IL-12B, CCL2, CCL5, and CXCL1. In addition, GPR84 activation triggered increased bacterial adhesion and phagocytosis in macrophages. The enhanced inflammatory response mediated by 6-OAU was not observed in GPR84−/− cells nor in macrophages treated with a selective GPR84 antagonist. Collectively, our results reveal that GPR84 functions as an enhancer of inflammatory signaling in macrophages once inflammation is established. Therefore, molecules that antagonize the GPR84 receptor may be potential therapeutic tools in inflammatory and metabolic diseases.

GPR84 is an orphan G-protein-coupled receptor that is expressed on immune cells and implicated in several inflammatory diseases. The validation of GPR84 as a therapeutic target is hindered by the narrow range of available chemical tools and consequent poor understanding of GPR84 pathophysiology. Here we describe the discovery and characterization of DL-175, a potent, selective, and structurally novel GPR84 agonist and the first to display significantly biased signaling across GPR84-overexpressing cells, primary murine macrophages, and human U937 cells. By comparing DL-175 with reported GPR84 ligands, we show for the first time that biased GPR84 agonists have markedly different abilities to induce chemotaxis in human myeloid cells, while causing similar levels of phagocytosis enhancement. This work demonstrates that biased agonism at GPR84 enables the selective activation of functional responses in immune cells and delivers a high-quality chemical probe for further investigation.

We identified TREM-1 as a major upstream proatherogenic receptor. We propose that TREM-1 activation orchestrates monocyte/macrophage proinflammatory responses and foam cell formation through coordinated and combined activation of CD36 and TLR4. Blockade of TREM-1 signaling may constitute an attractive novel and double-hit approach for the treatment of atherosclerosis.