G protein-coupled receptor 21 in macrophages: An in vitro study.

GPR21 is an orphan and constitutively active receptor belonging to the superfamily of G-Protein Coupled Receptors (GPCRs). GPR21 couples to the G family of G proteins and is expressed in macrophages. Studies of GPR21 knock-out mice indicated that GPR21 may be involved in promoting macrophage migration. The aim of this study was to evaluate the role of GPR21 in human macrophages, analyzing (i) its involvement in cell migration and cytokine release and (ii) the consequence of its pharmacological inhibition by using the inverse agonist GRA2. THP-1 cells were activated and differentiated into either M or M macrophages. GPR21 expression was evaluated at gene and protein level, the signalling pathway was investigated by an IP assay, and cytokine release by ELISA. Cell migration was detected by the Boyden chamber migration assay, performed on macrophages derived from both the THP-1 cell line and human peripheral blood monocytes. In addition, we compared the effect of the pharmacological inhibition of GPR21 with the effect of the treatment with a specific GPR21 siRNA to downregulate the receptor expression, thus confirming that GRA2 acts as an inverse agonist of GPR21. GRA2 does not affect cell viability at the tested concentrations, but significantly reduces the release of TNF-α and IL-1β from M macrophages. The analysis of the migratory ability highlighted opposite effects of GRA2 on M and M macrophages since it decreased M, while it promoted M cell migration. Therefore, the pharmacological inhibition of GPR21 could be of interest for pathological conditions characterized by low grade chronic inflammation.