Thrombin causes increased monocytic-cell adhesion to endothelial cells through a protein kinase C-dependent pathway

The coagulation protein thrombin has been shown to stimulate multiple endothelial-cell (EC) functions, including production of platelet-derived growth factor and of platelet-activating factor (PAF), and neutrophil adhesion. We have found that thrombin causes increased binding of monocytic cells (U937 cells and normal human monocytes) to cultured EC of various species. Maximum adhesion of monocytes to pig aortic EC occurred 6 h after thrombin treatment and remained elevated through 24 h. Stimulation of adherence by bovine alpha-thrombin was half-maximal at 15 units/ml, and reached a plateau at 50 units/ml. Catalytically inactive thrombin (phenylmethanesulphonyl fluoride-treated) had no effect on monocyte adhesion to EC. Heparin, but not the endotoxin antagonist polymyxin B, suppressed the stimulation of adhesion by thrombin without altering basal adhesion. Two lines of evidence suggested that protein kinase C (PKC) was involved in the intracellular signalling to increase monocyte adhesion to EC. First the PKC activator phorbol 12-myristate 13-acetate (PMA) stimulated monocytic-cell adhesion to EC at a dose consistent with stimulation of PKC (half-maximal response at 1-3 nM) and with a time course similar to that for thrombin stimulation (maximal by 4 h). Diacylglycerol, a physiological activator of PKC, also stimulated U937-cell adhesion to EC. Secondly, H7, a PKC inhibitor, completely blocked stimulation of monocyte adhesion to EC by thrombin or PMA. The structural analogue of H7, HA1004, which preferentially inhibits cyclic-AMP- and cyclic-GMP-dependent protein kinases, had no effect on stimulated monocyte adhesion. The PKC inhibitor also blocked the stimulation of monocyte adhesion to EC by interleukin-1 and endotoxin, but did not alter the basal level of monocyte binding to unstimulated EC. Thrombin stimulation of monocyte adhesion differed from the reported stimulation of neutrophil adhesion by thrombin in that the latter process reached a maximum in minutes rather than hours. In addition, neither PAF itself nor agents known to stimulate PAF production by EC, such as arachidonate and the Ca2+ ionophore A23187, had any effect on monocyte adhesion. These results demonstrate a PKC-dependent cytokine-like action of the coagulation protein thrombin in modulating monocytic-cell adhesion to EC, a phenomenon of potential importance in many pathological and physiological processes.