In this work, we analyzed the role of the PI3K-p70 S6 kinase (S6K) signaling cascade in the stimulation of endothelial cell proliferation. We found that inhibitors of the p42/p44 MAPK pathway (PD98059) and the PI3K-p70 S6K pathway (wortmannin, Ly294002, and rapamycin) all block thymidine incorporation stimulated by fetal calf serum in the resting mouse endothelial cell line 1G11. The action of rapamycin can be generalized, since it completely inhibits the mitogenic effect of fetal calf serum in

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... primary endothelial cell cultures (human umbilical vein endothelial cells) and another established capillary endothelial cell line (LIBE cells). The inhibitory effect of rapamycin is only observed when the inhibitor is added at the early stages of G 0 -G 1 progression, suggesting an inhibitory action early in G 1 . Rapamycin completely inhibits growth factor stimulation of protein synthesis, which perfectly correlates with the inhibition of cell proliferation. In accordance with its inhibitory action on protein synthesis, activation of cyclin D1 and p21 proteins by growth factors is also blocked by preincubation with rapamycin. Expression of a p70 S6K mutant partially resistant to rapamycin reverses the inhibitory effect of the drug on DNA synthesis, indicating that rapamycin action is via p70 S6K. Thus, in vascular endothelial cells, activation of protein synthesis via p70 S6K is an essential step for cell cycle progression in response to growth factors.