Abstract
Macrophage migration inhibitory factor (MIF) is a pro-inflammatory cytokine that was initially identified by its ability to inhibit the movement of macrophages. Cell migration is a highly complex process involving changes to the cytoskeleton and cell adhesion molecules, and is regulated by the Rho GTPases. A simple model using human monocytic U-937 cells to elicit the classic MIF response was implemented to examine the mechanism of MIF-induced migration inhibition. Our results demonstrate that MIF inhibits migration of these U-937 cells through a non-canonical receptor, CXCR4, in the absence of the putative primary MIF receptor CD74. Migration inhibition is dependent upon a series of temporal perturbations of the activities of the Rho GTPases: initial activation followed by subsequent inactivation of RhoA, inactivation of Rac1, and cyclic activation of Cdc42. MIF-mediated changes in the activities of the Rho GTPases jointly contributed to migration inhibition in these cells. Collectively, these data suggest that the MIF-mediated migration inhibition is mediated by the outcome of G-protein signaling, and in less adherent cells such as those of the monocyte/macrophage lineage, RhoA directly affects net translocation through its ability to induce cell body contraction. These findings demonstrate that CXCR4 can mediate MIF signaling in the absence of CD74 in addition to serving as a MIF co-receptor along with CD74. These results correlate MIF activity to specific and sequential Rho GTPase activity perturbations, and given that CXCR4 functions in numerous processes, suggests potential roles for the modulation of cell movement in those events including development, cell survival and viral infection.