Thromboxane prostanoid receptor stimulation induces shedding of the transmembrane chemokine CX3CL1 yet enhances CX3CL1-dependent leukocyte adhesion.

In atherosclerosis, chemokines recruit circulating mononuclear leukocytes to the vascular wall. A key factor is CX(3)CL1, a chemokine with soluble and transmembrane species that acts as both a chemoattractant and an adhesion molecule. Thromboxane A(2) and its receptor, TP, are also critical to atherogenesis by promoting vascular inflammation and consequent leukocyte recruitment. We examined the effects of TP stimulation on processing and function of CX(3)CL1, using CX(3)CL1-expressing human ECV-304 cells and primary human vascular endothelial cells. TP agonists promoted rapid shedding of cell surface CX(3)CL1, which was inhibited by pharmacological inhibitors or specific small interfering RNA targeting tumor necrosis factor-alpha-converting enzyme (TACE). Because it reduced cell surface CX(3)CL1, we predicted that TP stimulation would inhibit adhesion of leukocytes expressing the CX(3)CL1 cognate receptor but, paradoxically, saw enhanced adhesion. We questioned whether the enhanced ability of the remaining membrane-associated CX(3)CL1 to bind targets was caused by changes in its lateral mobility. Using fluorescence recovery after photobleaching, we found that plasmalemmal CX(3)CL1 was initially tethered but ultimately mobilized by TP agonists. TP stimulation provoked clustering of transmembrane CX(3)CL1 at sites of contact with adherent leukocytes. These data demonstrate that TP stimulation induces two distinct effects: a rapid cleavage of surface CX(3)CL1, thereby releasing the soluble chemoattractant, plus mobilization of the remaining transmembrane CX(3)CL1 to enhance the avidity of interactions with adherent leukocytes. The dual effect of TP allows CX(3)CL1 to recruit leukocytes to sites of vascular inflammation while enhancing their adhesion once recruited.