Distinct signaling pathways for MCP-1-dependent integrin activation and chemotaxis.

Transmigration of monocytes to the subendothelial space is the initial step of atherosclerotic plaque formation and inflammation. Integrin activation and chemotaxis are two important functions involved in monocyte transmigration. To delineate the signaling cascades leading to integrin activation and chemotaxis by monocyte chemoattractant protein-1 (MCP-1), we have investigated the roles of MAPK and Rho GTPases in THP-1 cells, a monocytic cell line. MCP-1 stimulated beta1 integrin-dependent, but not beta2 integrin-dependent cell adhesion in a time-dependent manner. MCP-1-mediated cell adhesion was inhibited by a MEK inhibitor but not by a p38-MAPK inhibitor. In contrast, MCP-1-mediated chemotaxis was inhibited by the p38-MAPK inhibitor but not by the MEK inhibitor. The inhibitor of Rho GTPase, C3 exoenzyme, and a Rho kinase inhibitor abrogated MCP-1-dependent chemotaxis but not integrin-dependent cell adhesion. Further, C3 exoenzyme and the Rho kinase inhibitor blocked MCP-1-dependent p38-MAPK activation. These data indicate that ERK is responsible for integrin activation, that p38-MAPK and Rho are responsible for chemotaxis mediated by MCP-1, and that Rho and the Rho kinase are upstream of p38-MAPK in MCP-1-mediated signaling. This study demonstrates that two distinct MAPKs regulate two dependent signaling cascades leading to integrin activation and chemotaxis induced by MCP-1 in THP-1 cells.