Molecular Characterization of the Cannabinoid-Mediated Migration of Monocytes.

2-Arachidonoylglycerol (2-AG), an endogenous ligand for the cannabinoid receptors CB1 and CB2, functions as a chemokine for monocyte migration. However, the molecular mechanism of its chemotactic effects is not clear. We found, consistent with previous data, that 2-AG induces the migration of differentiated but not undifferentiated monocytic cells in a dose-dependent manner. We first asked whether the expression of cannabinoid receptors changed during monocytic differentiation. Treatment with 1,25-(OH)₂vitamin D3, a potent inducer of monocyte differentiation, or with 2-AG did not alter the surface expression of the CB1 and CB2 receptors, indicating that signaling downstream of receptor ligation accounted for the observed effect on monocyte migration. In addition, treatment of differentiated monocytic cells with inhibitors for adenyl cyclase and rho kinase blocked the 2-AG-mediated migration, directly implicating these signaling molecules in monocyte motility. Upon eliminating the concentration gradient of 2-AG, the motility of the cells from the upper to lower compartment was sharply reduced, but not completely abrogated. This suggested that the chemotactic effect may not fully explain the observed change in cell migration. Of note, treatment of monocytes with 2-AG resulted in an enhanced secretion of the chemokines MCP-1 and IL-8. Moreover, exposure of the cells to 2-AG inhibited their migration towards MCP-1, while exposure to MCP-1 did not alter migration toward 2-AG. Taken together, our findings demonstrate that intracellular signaling cascades as well as induction of chemokine secretion contribute to the cannabinoid-mediated migration of monocytes.