THROMBIN STIMULATES PDGF PRODUCTION AND MONOCYTE ADHESION THROUGH DISTINCT INTRACELLULAR PATHWAYS IN HUMAN ENDOTHELIAL-CELLS

Thrombin stimulates multiple functions in cultured endothelial cells (EC), including an increase in cell surface adhesion sites for monocytes and the production of platelet-derived growth factor (PDGF). We have initiated studies to define the intracellular signaling pathways involved in these two thrombin-induced EC functions by focusing on the possible roles of the Na+-H+ antiporter and guanine nucleotide-binding proteins (G proteins). Amiloride suppressed thrombin-stimulated PDGF production by human aortic EC without affecting either basal PDGF production or overall protein synthesis. The steady-state mRNA levels of PDGF-A and PDGF-B chain were not reduced by amiloride. In replicate EC cultures, amiloride had no effect on thrombin-stimulated monocyte adhesion. In addition, thrombin induction of PDGF production, but not monocyte adhesion, was abrogated in the absence of extracellular sodium. Thrombin stimulation of both monocyte adhesion and PDGF production appeared to involve a pertussis toxin-insensitive G protein. Thrombin induced an increase in [S-35]guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) binding to human EC membranes. GTP-gamma-S, in the presence of a suboptimal concentration of thrombin, caused maximal stimulation of both monocyte adhesion and PDGF production. The effect of GTP-gamma-S on PDGF production was at the level of transcription. These results indicate that the EC is capable of responding to a pluripotent agonist such as thrombin through multiple signaling pathways, which converge and diverge to achieve differential cellular responses.