CAMP-DEPENDENT PROTEIN-KINASE INHIBITS THE MITOGENIC ACTION OF VASCULAR ENDOTHELIAL GROWTH-FACTOR AND FIBROBLAST GROWTH-FACTOR IN CAPILLARYENDOTHELIAL-CELLS BY BLOCKING RAF ACTIVATION

Proliferation of endothelial cells is regulated by angiogenic and anti angiogenic factors whose actions are mediated by complex interactions of multiple signaling pathways. Both vascular endothelial growth facto r (VEGF) and basic fibroblast growth factor (bFGF) stimulate cell prol iferation and activate the mitogen-activated protein kinase (MAPK) cas cade in bovine brain capillary endothelial (BBE) cells. We have extend ed these findings to show that both mitogens activate MAPK via stimula tion of Raf-1. Activation of Raf/MAPK is inhibited by increasing intra cellular cAMP levels pharmacologically or via stimulation of endogenou sly expressed beta-adrenergic receptors. Both VEGF- and bFGF-induced R af-1 activity are blocked in the presence of forskolin or 8-bromo-cAMP by 80%. The actions of increased cAMP appear to be mediated by cAMP-d ependent protein kinase (PKA), since treatment with H-89, a the specif ic inhibitor of PKA, reversed the inhibitory effect of elevated cAMP l evels on mitogen-induced cell proliferation and Raf/MAPK activation. M oreover, elevations in cAMP/PKA activity inhibit mitogen-induced cell proliferation. These findings demonstrate, in cultured endothelial cel ls, that the cAMP/PKA signaling pathway is potentially an important ph ysiological inhibitor of mitogen activation of the MAPK cascade and ce ll proliferation. (C) 1997 Wiley-Liss, Inc.