Tumor Necrosis Factor-α Potentiates RhoA-Mediated Monocyte Transmigratory Activity In Vivo at a Picomolar Level

OBJECTIVE—The serum level of tumor necrosis factor-α (TNF-α) is in the picomolar range under inflammatory conditions. We investigated whether these picomolar levels of TNF-α directly modulate the functional activities of circulating monocytes. METHODS AND RESULTS—In THP-1 monocytes treated with TNF-α (1 to 100 pmol/L/30 minutes), cytosolic RhoA small GTPase rapidly translocated to the plasma membrane via functionally active ezrin/radixin/moesin (ERM) complex, a cytoskeletal linker, and subsequent actin polymerization through NF-κB activation. The threonine phosphorylation of ERM was accomplished by the activation of TNF receptor type I (TNFRI) and signaling pathways involving PI3K and an atypical PKC; ie, PKCζ. The TNF-α-treated monocytes (10 pmol/L) displayed more potent and prolonged generation of GTP-bound RhoA in response to secondary stimulation with RhoA-activating monocyte chemoattractant protein-1 (MCP-1). Clearly, human circulating monocytes preconditioned by 10 pmol/L TNF-α augmented MCP-1–mediated chemotaxis and firm adhesion on VCAM-1 and ICAM-1 in vitro and ex vivo. The elevation of serum TNF-α (>5 pmol/L within 16 hours), which was introduced by intraperitoneal injection of mouse-specific TNF-α to C57/BL6 mice, enhanced the number of CD80+ monocytes transmigrating to the JE/MCP-1–injected intraperitoneal space. CONCLUSIONS—Picomolar concentrations of TNF-α in the bloodstream may prime the RhoA-dependent activities of circulating monocytes to enhance recruitment to active inflammatory foci.