p70 S6 kinase-mediated protein synthesis is a critical step for vascular endothelial cell proliferation

In this work, we analyzed the role of the PI3K-p70 S6 kinase (S6K) signalin g cascade in the stimulation of endothelial cell proliferation. We found th at inhibitors of the p42/p44 MAPK pathway (PD98059) and the PI3K-p70 S6K pa thway (wortmannin, Ly294002, and rapamycin) all block thymidine incorporati on stimulated by fetal calf serum in the resting mouse endothelial cell lin e 1G11. The action of rapamycin can be generalized, since it completely inh ibits the mitogenic effect of fetal calf serum in primary endothelial cell cultures(human umbilical vein endothelial cells) and another established ca pillary endothelial cell line (LIBE cells). The inhibitory effect of rapamy cin 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). Rapamyci n completely inhibits growth factor stimulation of protein synthesis, which perfectly correlates with the inhibition of cell proliferation. In accorda nce with its inhibitory action on protein synthesis, activation of cyclin D 1 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 th at rapamycin action is via p70 S6K, Thus, in vascular endothelial cells, ac tivation of protein synthesis via p70 S6K is an essential step for cell cyc le progression in response to growth factors.