The mitogenic effect of thrombin in vascular smooth muscle cells is largely due to basic fibroblast growth factor.

Mitogenesis induced by most polypeptide growth factors is mediated by either G-protein- or tyrosine kinase-linked receptor pathways. Thrombin, a potent mitogen for vascular smooth muscle cells, activates a G-protein-coupled receptor but also requires tyrosine kinase activity for its mitogenic effect (Weiss, R. H., and Nuccitelli, R. (1992) J. Biol. Chem. 267, 5608-5613). Since this growth factor elicits a synergistic effect on DNA synthesis when applied to cells concurrently with basic fibroblast growth factor (bFGF), we suspected that the two growth factors have points of convergence in their signaling pathways. We now show that when 1 unit/ml thrombin is removed after an incubation period of from 4 h to 15 min prior to 15 ng/ml bFGF addition, its synergistic effect with bFGF on mitogenesis in vascular smooth muscle cells is preserved. Furthermore, appearance of bFGF in the cellular lysate is maximal 1 h after the addition of 1 unit/ml thrombin. While monoclonal antibody to bFGF inhibits thrombin's mitogenic effect by 63% at 30 micrograms/ml, it lacks an inhibitory effect on platelet-derived growth factor-BB-induced mitogenesis. The inhibitory effect of bFGF antibody on thrombin's growth is completely reversed by the addition of bFGF. These data demonstrate that the presence of bFGF is essential for thrombin to exert its full mitogenic effect in vascular smooth muscle cells, providing an example of a system where a tyrosine kinase-linked growth factor receptor system can act as an essential intermediary in the mitogenic signaling pathway of a G-protein-coupled receptor.